Update cinder backup_driver to full class name

Legacy backup service support was recently dropped from cinder in
change I3ada2dee1857074746b1893b82dd5f6641c6e579 and we need to
adjust how we set the config option in devstack accordingly. This
updates the backup_driver option to specify a full class name instead
of only the module name.

Closes-Bug: #1794859

Change-Id: I3a72f38b564b8b83b233fccba7685833b6394d45

Author: melwitt

lib/cinder_backends/ceph

@@ -65,7 +65,7 @@ function configure_cinder_backend_ceph {
         sudo ceph -c ${CEPH_CONF_FILE} auth get-or-create client.${CINDER_BAK_CEPH_USER} mon "allow r" osd "allow class-read object_prefix rbd_children, allow rwx pool=${CINDER_BAK_CEPH_POOL}, allow rwx pool=${CINDER_CEPH_POOL}" | sudo tee ${CEPH_CONF_DIR}/ceph.client.${CINDER_BAK_CEPH_USER}.keyring
         sudo chown $(whoami):$(whoami) ${CEPH_CONF_DIR}/ceph.client.${CINDER_BAK_CEPH_USER}.keyring
 
-        iniset $CINDER_CONF DEFAULT backup_driver "cinder.backup.drivers.ceph"
+        iniset $CINDER_CONF DEFAULT backup_driver "cinder.backup.drivers.ceph.CephBackupDriver"
         iniset $CINDER_CONF DEFAULT backup_ceph_conf "$CEPH_CONF_FILE"
         iniset $CINDER_CONF DEFAULT backup_ceph_pool "$CINDER_BAK_CEPH_POOL"
         iniset $CINDER_CONF DEFAULT backup_ceph_user "$CINDER_BAK_CEPH_USER"