webadmin: disable rebalance action when its being started

Once the user clicks on 'Rebalance' action in volumes tab,
glusterfs takes few seconds to start the rebalance operation.
'Rebalance' action will be disabled immediately without
waiting for the result from glusterfs.

Change-Id: I961955f15310e5d0566019556e7f7e601e8ec957
Bug-Url: https://bugzilla.redhat.com/1021814
Signed-off-by: Kanagaraj M <kmayilsa@redhat.com>