gluster: event message for rebalance and remove brick complete

A suitable event message will be displayed when rebalance is
completed in a volume and when data migration for brick removal is complete.

Change-Id: If4da9730cd8c251bc5707df981204b092948fb6f
bug url: https://bugzilla.redhat.com/show_bug.cgi?id=1015016
Signed-off-by: ndarshan <dnarayan@redhat.com>