fix cache persistent default

Signed-off-by: Huamin Chen <hchen@redhat.com>

rbd/main.go

@@ -35,7 +35,7 @@ var (
 	driverName      = flag.String("drivername", "csi-rbdplugin", "name of the driver")
 	nodeID          = flag.String("nodeid", "", "node id")
 	containerized   = flag.Bool("containerized", true, "whether run as containerized")
-	metadataStorage = flag.String("metadatastorage", "", "metadata persistence method [node|k8s_configmap]")
+	metadataStorage = flag.String("metadatastorage", "node", "metadata persistence method [node|k8s_configmap]")
 )
 
 func main() {
@@ -50,6 +50,10 @@ func main() {
 		os.Exit(1)
 	}
 
+	if *metadataStorage != "node" && *metadataStorage != "k8s_configmap" {
+		glog.Errorf("invalid cache persistent option: %s", *metadataStorage)
+		os.Exit(1)
+	}
 	cp, err := util.NewCachePersister(*metadataStorage, *driverName)
 	if err != nil {
 		glog.Errorf("failed to define cache persistence method: %v", err)