CBQE-2195: fix is_cluster_rebalanced

Change-Id: I1c655768f4976504ae7164195f0a6684d5d1587e
Reviewed-on: http://review.couchbase.org/35644
Reviewed-by: Parag Agarwal <agarwal.parag@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>