Issue 4637: install xs-authserver

This playbook installs xs-authserver in a DXS. It uses:

- pip for install xs-authserver package from PyPI
- gunicorn as a WSGI HTTP server
- environment variables to configure
- systemd to control service

The server starts on ``0.0.0.0:5000``. To test use the FQDN, as http://schoolserver.local:5000/.

Related issue https://sugardextrose.org/issues/4637