CBQE-O: check that cluster balanced after 100% in RebalanceTask.check

Change-Id: I178f349231cfeb605bd4dfe98f88002e2afd01be
Reviewed-on: http://review.couchbase.org/39757
Reviewed-by: Parag Agarwal <agarwal.parag@gmail.com>
Tested-by: buildbot <build@couchbase.com>