webadmin: Volume rebalance status popup button caption changed

Changed the button caption of volume rebalance status popup from 'Stop Rebalance' to 'Stop' to maintain
consistency along the button names in the popups.

Change-Id: I907ba9b53dbff901cb0964ea90d36f0e66b7fedc
Bug-Url: https://bugzilla.redhat.com/1029484
Signed-off-by: Karnan TC <kchidamb@redhat.com>