glusterd/snapshot : Fail the snapshot create operation if geo-rep is running.

As one of the recommandations for taking a snapshot is not to have
an active geo-replication session, its better to display an error
saying session is active when snapshot create command is issued.

Change-Id: I94593dbd2659610e033ca316176dda1ac8dc5ce6
BUG: 1129038
Signed-off-by: Sachin Pandit <spandit@redhat.com>
Reviewed-on: http://review.gluster.org/8461
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Rajesh Joseph <rjoseph@redhat.com>
Reviewed-by: Atin Mukherjee <amukherj@redhat.com>
Reviewed-by: Vijaikumar Mallikarjuna <vmallika@redhat.com>
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Tested-by: Krishnan Parthasarathi <kparthas@redhat.com>