CBQE-0: fix rest_client.rebalance_reached()

Change-Id: Iad0f176a42c2871965330dc76db47aff0d246beb
Reviewed-on: http://review.couchbase.org/36412
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>