MB-100: fix TimeoutError in start_stop_rebalance_in_out

Change-Id: Ie8e45ed55a0954e1b2ea5182e54bdc833859fc09
Reviewed-on: http://review.couchbase.org/24930
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>