NCBC-640: Rename CouchbaseCluster to Cluster

Change-Id: Ia96fe4ec96aba69a4a3d10506d172c966afa64a4
Reviewed-on: http://review.couchbase.org/41742
Reviewed-by: Jeffry Morris <jeffrymorris@gmail.com>
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>