webadmin: action to stop brick removal operation

Intoduced an action group for brick removal operations.
Modified existing 'Remove' action to 'Start' and moved under
the new 'Remove' action group.

Added a new action 'Stop' to abort the brick removal operation.
On clicking this action, a confirmation will shown to make sure the user
actually wants to stop.

Change-Id: Ia1d1653e168aa00b6b4c17d4608f5d96558aa462
Signed-off-by: Kanagaraj M <kmayilsa@redhat.com>