Tighten permissions on Etherpad settings file

The file in which our Etherpad settings reside is templated with
sensitive data like an API key and DB password. Remove the world
readable bit from it, and also drop user/group write perms while
we're at it. Also switch the service's effective GID to match its
UID and make sure the config's ownership is set accordingly.

Change-Id: I65b70237b4bc8f4e63aa0b717702c124e01ed777

playbooks/roles/etherpad/tasks/main.yaml

@@ -89,6 +89,9 @@
   template:
     src: settings.json.j2
     dest: /etc/etherpad/settings.json
+    owner: 5001
+    group: 5001
+    mode: '0440'
 
 - name: Clean up from old ep_headings hack
   file:

zuul.d/docker-images/etherpad.yaml

@@ -9,6 +9,8 @@
         - context: docker/etherpad
           target: production
           repository: opendevorg/etherpad
+          build_args:
+            - EP_GID=5001
     files: &etherpad_files
       - docker/etherpad/