Fix up 'Persist nova libvirt secrets in a Docker volume'

Follow up fix for Ia7e923dddb77ff6db3c9160af931354a2b305e8d, which
broke the cephadm jobs.

Change-Id: Ieb39b41a6f493bd00c687610ba043a1b4e5945e7
Related-Bug: #1821696

ansible/roles/nova-cell/templates/nova-libvirt.json.j2

@@ -47,14 +47,14 @@
             "source": "{{ container_config_directory }}/secrets",
             "dest": "/etc/libvirt/secrets",
             "owner": "root",
-            "perm": "0600"
+            "perm": "0600",
+            "merge": true
         }{% endif %}{% if nova_backend == "rbd" %},
         {
             "source": "{{ container_config_directory }}/ceph.conf",
             "dest": "/etc/ceph/ceph.conf",
             "owner": "nova",
-            "perm": "0600",
-            "merge": true
+            "perm": "0600"
         }{% endif %}
     ]
 }