Switch this over now that xqueue's change has landed

CHANGELOG.md

 - Role: XQueue
   - Convert to a yaml config (instead of xqueue.auth.json and xqueue.env.json we get xqueue.yml and it lives by default in /edx/etc/xqueue.yml like standard IDAs)
   - Add XQUEUE_DEFAULT_FILE_STORAGE so that you can specify S3 or Swift in your config
+  - XQUEUE_SETTINGS now prefers production.py over aws_settings.py
 
 - Role: credentials
   - Set `LANGUAGE_COOKIE_NAME` so that Credentials will use the global language cookie.

playbooks/roles/xqueue/defaults/main.yml

 # variables common to the xqueue role, automatically loaded
 # when the role is included
 ---
-XQUEUE_SETTINGS: 'aws_settings'
+XQUEUE_SETTINGS: 'production'
 
 XQUEUE_NGINX_PORT: 18040
 XQUEUE_GUNICORN_WORKERS_EXTRA: ""