glusterd-volgen: Enable open-behind based on op-version

This patch enables the open-behind by default only when the op-version
allows it. Also the volume op-version calculations take account of this
enablement.

Change-Id: Idf7a3c274ec4828aafc815cdd1df829ecb853354
BUG: 954256
Signed-off-by: Kaushal M <kaushal@redhat.com>
Reviewed-on: http://review.gluster.org/4866
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Jeff Darcy <jdarcy@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>