engine: fail stop volume if rebalance is running

'Stop' gluster volume is not allowed, if there
is a rebalance operation is going on that volume.

Change-Id: I6deb52b358b2ef78c5f41df29d866085bdedb371
Bug-Url: https://bugzilla.redhat.com/1008900
Signed-off-by: Kanagaraj M <kmayilsa@redhat.com>