gverify/glusterd : Added check to warn the user of existing files in slave volume

Change-Id: I41c9fb44613386189d9f3c090729f4ded8e4dea8
BUG: 990997
Signed-off-by: Avra Sengupta <asengupt@redhat.com>
Signed-off-by: Venky Shankar <vshankar@redhat.com>
Reviewed-on: http://review.gluster.org/5453
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Reviewed-by: Anand Avati <avati@redhat.com>