glusterd: update volinfo->subvol_count during remove-brick operation.

Problem:
In glusterd_op_remove_brick(), volinfo->subvol_count was getting updated only if
the replica count was reduced due to which subvol_matcher_verify() gave false
errors under certain scenarios (see bug description).

Fix:
updated subvol_count for every remove-brick operation.

Change-Id: Id72691e2bda1c624cd7d8cae92f6bf32c101a6d3
BUG: 1120647
Signed-off-by: Ravishankar N <ravishankar@redhat.com>
Reviewed-on: http://review.gluster.org/8326
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Atin Mukherjee <amukherj@redhat.com>
Reviewed-by: Kaushal M <kaushal@redhat.com>
doc : man page for snapshot commands

This patch also contains few modifications in admin documentation.

Change-Id: I7bc2a88e6cbcfe81dcfafc2956f5b7c5524b0f0b
BUG: 1122399
Signed-off-by: Sachin Pandit <spandit@redhat.com>
Reviewed-on: http://review.gluster.org/8357
Reviewed-by: Avra Sengupta <asengupt@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Kaushal M <kaushal@redhat.com>