glusterd/snapshot: Check if LVM device path exists before delete.

Check if the LV is present before deleting the LV. In case where
the LV is absent (already deleted?), need not fail the snap delete
operation.

Also check if the LV is mounted before trying umount. In case it
isn't umounted, only remove the LV.

Change-Id: I0f5b2674797299d8748c6fac5b091f0caba65ca4
BUG: 1104714
Signed-off-by: Avra Sengupta <asengupt@redhat.com>
Reviewed-on: http://review.gluster.org/8954
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Tested-by: Krishnan Parthasarathi <kparthas@redhat.com>