Unable to add server groups with pre-existing nodeselectors

The operator takes ServerGroup tags from the CouchbaseCluster spec and applies them as nodeSelectors on the Pod.  Under normal circumstances this simply results in multiple nodeSelectors being present on the Pod.  However in this case the 'example' selector is applied first like so:

  nodeSelector:

    node.example.com/role: couchbase

Then when a new serverGroup is added, the Operator tries to convert this as yet another nodeSelector alongside this:

  nodeSelector:

    node.example.com/role: couchbase

    topology.kubernetes.io/zone: eu-central-1a 

But we never get to this state because the Operator doesn't like that there is an existing nodeSelector(node.example) that was not created via serverGroup (bug). And so the fix is to not require pre-existing node selectors to conform to serverGroup style labels.