The snapshot name returned by rbd_add_parse_args() just gets saved
in the rbd_dev eventually.  So just do that inside that function and
do away with the snap_name argument, both in rbd_add_parse_args()
and rbd_dev_set_mapping().
Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>