CBQE-1149: check reb status before killing memcached

handle situation when rebalance failed before killing memcached

Change-Id: I58000df21aec1c758bf7406ebac4b6b110c1ba3b
Reviewed-on: http://review.couchbase.org/25262
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>