Support v1alpha1.ScyllaDBCluster registration with global ScyllaDB Manager instance

[rzetelskik]

Acceptance criteria

• Create ScyllaDBManagerClusterRegistration objects for ScyllaDBClusters with a predefined label
  • Delete the internal object in case of ScyllaDBCluster deletion or predefined label removal: Scylla Manager controller should remove clusters from manager state on object deletion #1930
• Modify ScyllaDBCluster controller to not propagate the predefined label to ScyllaDBDatacenters
• Extend ScyllaDBManagerClusterRegistration controller to support ScyllaDBClusters
  • Includes auth token management
  • Includes blocking until the cluster is available
  • Includes finalizer for deregistering ScyllaDBClusters
• Add an E2E test for registering and deregistering ScyllaDBClusters with global ScyllaDB Manager instance.