[CX] Don't skip metadata replica addition on cancelled rebalance-in

As observed in CBSE-7278, a cancelled rebalance-in left an analytics cluster without any metadata replicas. Upon attempt to failover & remove the remaining node, the cluster becomes permanently unusable without removing all analytics nodes from the cluster and re-adding the service, which will result in an uninitialized analytics cluster.

On rebalance-in, we should attempt to add a metadata replica to protect ourselves from this scenario.