Do not need to check clusterKey if replicationEndpoint is provided when adding a peer

The clusterKey check is done in HBASE-19630, which is part of the work for HBASE-19397.

In HBASE-19630 we claim that we always check clusterKey when adding a peer at RS side, but this is not true, as clusterKey could be null. And it will be strange that if we implement a ReplicationEndpoint to kafka and we still need to provide a cluster key in the hbase format.