glusterd: remove-brick:Allow simultaneous removal of multiple subvolumes.

Currently, remove-brick supports removal of only one distributed
stripe/ replica pair at a time. Fix it to support removal of multiple
pairs. This is consistent with add-brick behaviour which supports adding
multiple stripe/replica pairs simultaneously.

Removal is successful irrespective of the order of the bricks given at
the CLI, as long as the bricks are from the same subvolume(s).

Change-Id: I7c11c1235ce07b124155978b9d48d0ea65396103
BUG: 974007
Signed-off-by: Ravishankar N <ravishankar@redhat.com>
Reviewed-on: http://review.gluster.org/5210
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>