glusterd: Use volume op-versions during volgen

  Backport of '3af61d6 glusterd: Use volume op-versions during volgen'
  from master

Instead of using the cluster op-version, volume op-version is used to
enable open-behind during volgen. For doing this, the volume op-versions
are updated before regenerating the volfiles.

BUG: 990830
Change-Id: I07e4a34004816c803fcbb3ee1ddd4b1e4c3a8006
Signed-off-by: Kaushal M <kaushal@redhat.com>
Reviewed-on: http://review.gluster.org/5831
Reviewed-by: Krutika Dhananjay <kdhananj@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>