webadmin: Adding warning messages to remove bricks

A warning message will be shown in the confirmation
dialog while remove brick(s) from a volume.

Change-Id: Ibf75ddb6f9f45fcc54004ff4b07c6c0ed9c4c6fc
Bug-Url: https://bugzilla.redhat.com/885757
Signed-off-by: Kanagaraj M <kmayilsa@redhat.com>