webadmin: action to retain the bricks being removed

Added a new 'Retain' action to 'Remove' action group in the Bricks
subtab.

  On clicking, a confirmation will be shown with the list of bricks
which are being removed. Upon the confirmation, the bricks will be
retained with the gluster volume.

Change-Id: I6b2c51bcd09dca6f8201c8caf265334e893cb0cd
Signed-off-by: Ramesh Nachimuthu <rnachimu@redhat.com>