exit with reason stopped if rebalance is stopped

Not just stop as we did while running pre-rebalance janitor.

Change-Id: I46010b77501df1a9504481d75035b41094375835
Reviewed-on: http://review.couchbase.org/39693
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>