Mount mnt folder when enabled cinder nfs backend

Change-Id: Ib3d0067ce4bc90245664a2fff337d4e71e4214c1

ansible/roles/nova/defaults/main.yml

@@ -31,7 +31,7 @@ nova_services:
       - "/etc/localtime:/etc/localtime:ro"
       - "kolla_logs:/var/log/kolla"
       - "nova_compute:/var/lib/nova"
-      - "/var/lib/nova/mnt:/var/lib/nova/mnt:shared"
+      - "{% if enable_cinder_backend_nfs | bool %}/var/lib/nova/mnt:/var/lib/nova/mnt:shared{% endif %}"
       - "heka_socket:/var/lib/kolla/heka/"
   placement-api:
     container_name: "placement_api"