CBQE-0: ignore mixed_cluster check for elasticsearch

Change-Id: I8170a1b86b1fea90cb8753a215bd864c7f194114
Reviewed-on: http://review.couchbase.org/27238
Reviewed-by: Tommie McAfee <tommie@couchbase.com>
Tested-by: Tommie McAfee <tommie@couchbase.com>