glusterd: op-version check for brickops.

cluster op-version must be atleast 4 for add/remove brick to proceed.
This change is required for the new afr-changelog xattr changes that
will be done for glusterFS 3.6 (http://review.gluster.org/#/c/7155/).

In add-brick, the check is done only when replica count is increased
because only that will affect the AFR xattrs.

In remove-brick, the check is unconditional failing which there will be
inconsistencies in the client xlator names amongst the volfiles of
different peers.

Change-Id: If981da2f33899aed585ab70bb11c09a093c9d8e6
BUG: 1066778
Signed-off-by: Ravishankar N <ravishankar@redhat.com>
Reviewed-on: http://review.gluster.org/7122
Reviewed-by: Kaushal M <kaushal@redhat.com>
Reviewed-by: Pranith Kumar Karampuri <pkarampu@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>