webadmin: enable start/stop rebalance based on status

'Start' rebalance action will be enabled if the
task status of the selected volume(s) is not 'STARTED'.

'Stop' rebalance action will be enabled if only one volume
is selected and the task is REBALANCE and the status is 'STARTED'.

Change-Id: Id01a605cfd3a59f3e3b584144f8f7589f0922f1e
Signed-off-by: Kanagaraj M <kmayilsa@redhat.com>