cli: Better message on rebalance/remove-brick stop

A better message is displayed when rebalance or remove-brick is stopped,
as migration may be still in progress.

Change-Id: I5f301cc66f349a1a85245f7d7508bf746756bdea
BUG: 923529
Signed-off-by: Kaushal M <kaushal@redhat.com>
Reviewed-on: http://review.gluster.org/4695
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>