APM: remove "output" element and add elasticsearchRef

[barkbay]

This PR adds the elasticsearchRef to the APM server object and remove the "output" section:

apiVersion: apm.k8s.elastic.co/v1alpha1
kind: ApmServer
metadata:
  name: apm-server-sample
spec:
  version: "7.2.0"
  nodeCount: 1
  elasticsearchRef:
    name: "elasticsearch-sample"

Internally the association controller is still managing a section elasticsearch to inject the connection information, exactly as it is done for Kibana:

spec:
  elasticsearch:
    auth:
      secret:
        key: default-apm-server-sample-apm-user
        name: apm-server-sample-elastic-internal-apm
    hosts:
    - https://elasticsearch-sample-es-http.default.svc:9200
    ssl:
      certificateAuthorities:
        secretName: elasticsearch-sample-es-http-certs-public
  elasticsearchRef:
    name: elasticsearch-sample

If it can be backported in 0.9. it would allow us to avoid a user change later.

[thbkrkr]

LGTM. Just left one tiny nitpick.

[thbkrkr]

Nit: we could reuse the ref variable: esNamespace := ref.Namespace. I may be a bit rigid but I would appreciate to have the same block of code with the same namings in the 2 controllers.

esRef := as.Spec.Output.Elasticsearch.ElasticsearchRef
if esRef == nil {
	return nil
}

if esRef.Namespace == "" {
	// no namespace provided: default to the APM Server namespace
	esRef.Namespace = as.Namespace
}

[barkbay]

Jenkins test this please

[anyasabo]

Jenkins test this please

[barkbay]

Jenkins test this please