Expect shutdown exit msgs in single vbucket mover subprocesses.

So that there's less CRASH REPORT noise in the logs when rebalance
gets stopped.

Change-Id: I624060ca4e80ab7c46bb4050126fb83d675f83ae
Reviewed-on: http://review.couchbase.org/40439
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>