glusterd/snapshot : Remove the barrier key from snap volinfo
before generating brick volfile.

Problem : During snapshot creation if I/O is in progress,
then barrier value is enabled. Hence during snapshot create
and in-turn snapshot restore the barrier value is set to enable.
Because of this further I/O on the mount point fails.

Solution : Remove the barrier key from newly created snap volinfo
before generating the brick volfiles.

Change-Id: I180b3adfbb364159fd353b2d0fb630e004099aa5
BUG: 1098487
Signed-off-by: Sachin Pandit <spandit@redhat.com>
Reviewed-on: http://review.gluster.org/7892
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Tested-by: Krishnan Parthasarathi <kparthas@redhat.com>