[DOCS] Correct secret uuid in Ceph example

Previously the UUID of the cluster's filesystem instead of the UUID
generated for the libvirt secret store was used by mistake.

The result creates a mismatch where cinder will tell nova to use the
fsid UUID in libvirt, while libvirt has the secret stored under the
ceph secret uuid.

Change-Id: Ifef6f2da8a6595fa4acb7c50bded25dbe3e13679

etc/openstack_deploy/user_variables.yml.prod-ceph.example

@@ -37,5 +37,5 @@ cinder_backends:
     rbd_store_chunk_size: 8
     volume_backend_name: rbddriver
     rbd_user: "{{ cinder_ceph_client }}"
-    rbd_secret_uuid: "{{ fsid }}"
+    rbd_secret_uuid: "{{ cinder_ceph_client_uuid }}"
     report_discard_supported: true