Mb-100: fix TimeoutError in start_stop_rebalance_in_out

http://qa.hq.northscale.net/view/2.0.1/job/ubuntu-64-2.0-new-rebalance-tests-P0/358/consoleFull

Change-Id: Ie03ad8014aa9d5f6cc88ca8bade06be2b1e42d41
Reviewed-on: http://review.couchbase.org/24660
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>