Change Cinder api_class setting to backend

Castellan switched the `api_class` config option to `backend` in commit
8980bf7da55dd084ad84c84534fe937f0d43b9c0. The old setting will still be
recognized for now, but we should switch to using the new, correct config
option.

Change-Id: I5e46c738531d5d56777e91a00f4cee9531356f2e

Author: stmcginnis

lib/cinder

@@ -243,7 +243,7 @@ function configure_cinder {
     iniset $CINDER_CONF DEFAULT periodic_interval $CINDER_PERIODIC_INTERVAL
     iniset $CINDER_CONF DEFAULT my_ip "$HOST_IP"
 
-    iniset $CINDER_CONF key_manager api_class cinder.keymgr.conf_key_mgr.ConfKeyManager
+    iniset $CINDER_CONF key_manager backend cinder.keymgr.conf_key_mgr.ConfKeyManager
 
     if is_service_enabled c-vol && [[ -n "$CINDER_ENABLED_BACKENDS" ]]; then
         local enabled_backends=""