Add annotation filter to Ambassador Host Source

This change makes the Ambassador Host source respect the External-DNS annotationFilter allowing for an Ambassador Host resource to specify what External-DNS deployment to use when there are multiple External-DNS deployments within the same cluster. Before this change if you had two External-DNS deployments within the cluster and used the Ambassador Host source the first External-DNS to process the resource will create the record and not the one that was specified in the filter annotation.

I added the `filterByAnnotations` function so that it matched the same way the other sources have implemented annotation filtering. I didn't add the controller check only because I wanted to keep this change to implementing the annotationFilter.

I added Endpoint tests to validate that the filterByAnnotations function works as expected. Again these tests were based of the Endpoint tests that other sources use. To keep the tests simpler I only allow for a single load balancer to be used.

Example: Create two External-DNS deployments 1 public and 1 private and set the Ambassador Host to use the public External-DNS using the annotation filter.

```
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: external-dns-private
spec:
  strategy:
    type: Recreate
  selector:
    matchLabels:
      app: external-dns-private
  template:
    metadata:
      labels:
        app: external-dns-private
      annotations:
        iam.amazonaws.com/role: {ARN} # AWS ARN role
    spec:
      serviceAccountName: external-dns
      containers:
      - name: external-dns
        image: k8s.gcr.io/external-dns/external-dns:latest
        args:
        - --source=ambassador-host
        - --domain-filter=example.net # will make ExternalDNS see only the hosted zones matching provided domain, omit to process all available hosted zones
        - --provider=aws
        - --policy=upsert-only # would prevent ExternalDNS from deleting any records, omit to enable full synchronization
        - --aws-zone-type=private # only look at public hosted zones (valid values are public, private or no value for both)
        - --registry=txt
        - --txt-owner-id= {Hosted Zone ID} # Insert Route53 Hosted Zone ID here
        - --annotation-filter=kubernetes.io/ingress.class in (private)
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: external-dns-public
spec:
  strategy:
    type: Recreate
  selector:
    matchLabels:
      app: external-dns-public
  template:
    metadata:
      labels:
        app: external-dns-public
      annotations:
        iam.amazonaws.com/role: {ARN} # AWS ARN role
    spec:
      serviceAccountName: external-dns
      containers:
      - name: external-dns
        image: k8s.gcr.io/external-dns/external-dns:latest
        args:
        - --source=ambassador-host
        - --domain-filter=example.net # will make ExternalDNS see only the hosted zones matching provided domain, omit to process all available hosted zones
        - --provider=aws
        - --policy=upsert-only # would prevent ExternalDNS from deleting any records, omit to enable full synchronization
        - --aws-zone-type= # only look at public hosted zones (valid values are public, private or no value for both)
        - --registry=txt
        - --txt-owner-id= {Hosted Zone ID} # Insert Route53 Hosted Zone ID here
        - --annotation-filter=kubernetes.io/ingress.class in (public)
---
apiVersion: getambassador.io/v3alpha1
  kind: Host
  metadata:
    name: your-hostname
    annotations:
      external-dns.ambassador-service: emissary-ingress/emissary
      kubernetes.io/ingress.class: public
  spec:
		acmeProvider:
      authority: none
		hostname: your-hostname.example.com
```

Fixes #2632

source/ambassador_host.go

@@ -57,6 +57,7 @@ type ambassadorHostSource struct {
 	dynamicKubeClient      dynamic.Interface
 	kubeClient             kubernetes.Interface
 	namespace              string
+	annotationFilter       string
 	ambassadorHostInformer informers.GenericInformer
 	unstructuredConverter  *unstructuredConverter
 }
@@ -66,7 +67,9 @@ func NewAmbassadorHostSource(
 	ctx context.Context,
 	dynamicKubeClient dynamic.Interface,
 	kubeClient kubernetes.Interface,
-	namespace string) (Source, error) {
+	namespace string,
+	annotationFilter string,
+) (Source, error) {
 	var err error
 
 	// Use shared informer to listen for add/update/delete of Host in the specified namespace.
@@ -84,6 +87,7 @@ func NewAmbassadorHostSource(
 
 	informerFactory.Start(ctx.Done())
 
+	// wait for the local cache to be populated.
 	if err := waitForDynamicCacheSync(context.Background(), informerFactory); err != nil {
 		return nil, err
 	}
@@ -97,6 +101,7 @@ func NewAmbassadorHostSource(
 		dynamicKubeClient:      dynamicKubeClient,
 		kubeClient:             kubeClient,
 		namespace:              namespace,
+		annotationFilter:       annotationFilter,
 		ambassadorHostInformer: ambassadorHostInformer,
 		unstructuredConverter:  uc,
 	}, nil
@@ -110,7 +115,8 @@ func (sc *ambassadorHostSource) Endpoints(ctx context.Context) ([]*endpoint.Endp
 		return nil, err
 	}
 
-	endpoints := []*endpoint.Endpoint{}
+	// Get a list of Ambassador Host resources
+	var ambassadorHosts []*ambassador.Host
 	for _, hostObj := range hosts {
 		unstructuredHost, ok := hostObj.(*unstructured.Unstructured)
 		if !ok {
@@ -122,7 +128,18 @@ func (sc *ambassadorHostSource) Endpoints(ctx context.Context) ([]*endpoint.Endp
 		if err != nil {
 			return nil, err
 		}
+		ambassadorHosts = append(ambassadorHosts, host)
+	}
+
+	// Filter Ambassador Hosts
+	ambassadorHosts, err = sc.filterByAnnotations(ambassadorHosts)
+	if err != nil {
+		return nil, errors.Wrap(err, "failed to filter Ambassador Hosts")
+	}
 
+	endpoints := []*endpoint.Endpoint{}
+
+	for _, host := range ambassadorHosts {
 		fullname := fmt.Sprintf("%s/%s", host.Namespace, host.Name)
 
 		// look for the "exernal-dns.ambassador-service" annotation. If it is not there then just ignore this `Host`
@@ -274,3 +291,35 @@ func newUnstructuredConverter() (*unstructuredConverter, error) {
 
 	return uc, nil
 }
+
+// Filter a list of Ambassador Host Resources to only return the ones that
+// contain the required External-DNS annotation filter
+func (sc *ambassadorHostSource) filterByAnnotations(ambassadorHosts []*ambassador.Host) ([]*ambassador.Host, error) {
+	// External-DNS Annotation Filter
+	labelSelector, err := metav1.ParseToLabelSelector(sc.annotationFilter)
+	if err != nil {
+		return nil, err
+	}
+
+	selector, err := metav1.LabelSelectorAsSelector(labelSelector)
+	if err != nil {
+		return nil, err
+	}
+
+	// empty filter returns original list of Ambassador Hosts
+	if selector.Empty() {
+		return ambassadorHosts, nil
+	}
+
+	// Return a filtered list of Ambassador Hosts
+	filteredList := []*ambassador.Host{}
+	for _, host := range ambassadorHosts {
+		annotations := labels.Set(host.Annotations)
+		// include Ambassador Host if its annotations match the annotation filter
+		if selector.Matches(annotations) {
+			filteredList = append(filteredList, host)
+		}
+	}
+
+	return filteredList, nil
+}