CBQE-0 :: fix verify_unacked_bytes_all_buckets when we fail master node in failover tests

Change-Id: I341a0098a1466d68870e60993237978f4e3eb1b0
Reviewed-on: http://review.couchbase.org/39415
Reviewed-by: Thuan Nguyen <soccon@gmail.com>
Reviewed-by: Parag Agarwal <agarwal.parag@gmail.com>
Tested-by: Parag Agarwal <agarwal.parag@gmail.com>