Write readme.txt into old log directories

This should help operators find the new log files. We do have them
documented, but not everybody reads every word in the docs :)

The readme creation has ignore_errors: true so that if the directory
isn't present at all (e.g. on deployed server environments, which
don't have openstack packages installed), we don't fail the deployment
when we're not able to create the readme.

Change-Id: I6b36db7b7ce8b3e4da566eb7828d0c3b8646a14f
Partial-Bug: #1730957

docker/services/ovn-dbs.yaml

@@ -198,6 +198,13 @@ outputs:
           with_items:
             - /var/log/containers/openvswitch
             - /var/lib/openvswitch/ovn
+        - name: openvswitch logs readme
+          copy:
+            dest: /var/log/openvswitch/readme.txt
+            content: |
+              Log files from openvswitch containers can be found under
+              /var/log/containers/openvswitch.
+          ignore_errors: true
       upgrade_tasks:
         - name: Stop and disable ovn-northd service
           tags: step2