make sure that all rebalance subprocesses get killed before new

.. janitor_agent is started after the old one was brutally killed

Change-Id: Iee03330b747c9a5227e27ad8c288ab8d0beaec68
Reviewed-on: http://review.couchbase.org/38586
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>