Fix product backend usages controller

[eguzki]

what

Fixes Jira issue: https://issues.redhat.com/browse/THREESCALE-8401

verification steps

Create 2 backends CR

k apply -f - <<EOF
---
apiVersion: capabilities.3scale.net/v1beta1
kind: Backend
metadata:
  name: backend-a
spec:
  name: "Operated Backend A"
  systemName: "backenda"
  privateBaseURL: "http://httpbin.org"
---
apiVersion: capabilities.3scale.net/v1beta1
kind: Backend
metadata:
  name: backend-b
spec:
  name: "Operated Backend B"
  systemName: "backendb"
  privateBaseURL: "http://httpbin.org"
EOF

Create product linking to the previous backend usages

k apply -f - <<EOF
---
apiVersion: capabilities.3scale.net/v1beta1
kind: Product
metadata:
  name: product1
spec:
  name: "OperatedProduct 1"
  backendUsages:
    backenda:
      path: /A
    backendb:
      path: /B
EOF

Delete Backend B from product's backend usage list and also delete the backend B from the backend list via UI

Add an annotation to the product to force reconciliation loop.

k annotate product product1 hello=1

Check the operator does not crash. Check the Product CR gets into a sync: false state:

k get product product1 -o jsonpath='{.status}' | yq e -P 
conditions:
  - lastTransitionTime: "2022-05-04T13:51:13Z"
    message: 'Task failed SyncBackendUsage: Backend SystemName backendb not found in 3scale backend index'
    status: "True"
    type: Failed
  - lastTransitionTime: "2022-05-04T13:48:38Z"
    status: "False"
    type: Invalid
  - lastTransitionTime: "2022-05-04T13:48:38Z"
    status: "False"
    type: Orphan
  - lastTransitionTime: "2022-05-04T13:51:13Z"
    status: "False"
    type: Synced
observedGeneration: 2
providerAccountHost: https://3scale-supertest-admin.3scale.net

One of the condition messages gives the clue: Task failed SyncBackendUsage: Backend SystemName backendb not found in 3scale backend index

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[Patryk-Stefanski]

Code change looks good. Condition type failed shows the error after removing backend usage in product and backends list through UI. The operator continues to run.
/lgtm