mbp-968: Add support for ACM ManagedClusters

[mlorenzofr]

These changes enable the import of pre-existing Openshift clusters using ACM.

Importing existing clusters

The pattern supports importating pre-existing Openshift clusters into the Hub cluster, converting them into Managed Clusters.

1. Copy the kubeconfig file of the cluster you want to import to your local system.

2. In the values-secret.yaml file, add a new secret with the contents of the kubeconfig file.

     - name: kubeconfig-spoke
       vaultPrefixes:
       - hub
       fields:
       - name: content
         path: ~/.kube/kubeconfig-ztvp-spoke

3. In the values-hub.yaml file, add a new entry in the clusterGroup.managedClusterGroups key.

     managedClusterGroups:
       exampleRegion:
         name: group-one
         acmlabels:
           - name: clusterGroup
             value: group-one
         helmOverrides:
           - name: clusterGroup.isHubCluster
             value: false

4. Also in the values-hub.yaml file, add your cluster definition in the acmManagedClusters.clusters key.

   acmManagedClusters:
     clusters:
       - name: ztvp-spoke-1
         clusterGroup: group-one
         labels:
           cloud: auto-detect
           vendor: auto-detect
         kubeconfigVaultPath: secret/data/hub/kubeconfig-spoke

[ghost]

Tested with a rhdp 3 control plane cluster as hub and a clusterbot aws 4.19 spoke.

[sabre1041]

LGTM