webadmin : added volume rebalance start action

Added a new action group 'Rebalace' to volumes tab.
'Start' rebalance volume is added to the group.

Change-Id: Idf8e3478abaace95adc2a565cb3e7c95dd0e7780
Signed-off-by: Anmol Babu <anbabu@redhat.com>
Signed-off-by: Kanagaraj M <kmayilsa@redhat.com>