Automate Services with systemd

Production web traffic must be served by node3 and node4 only. Someone started Apache on the dev box by hand and the test box runs an `atd` nobody needs. Fix all of it with playbooks and add a small WebAsha heartbeat service on dev.

Tasks

1. Write ~/ansible/services.yml: httpd started and enabled at boot on the webservers; stopped and disabled on dev and test.
2. In the same project, write ~/ansible/atd.yml that stops and disables atd on the test group only.
3. Write ~/ansible/webpage.yml: deploy /var/www/html/index.html with the content Welcome to WebAsha prod on the webservers, and restart httpd through a handler only when the page changes. curl http://node3 from control must show the page.
4. Write ~/ansible/heartbeat.yml: on dev only, install the unit file /etc/systemd/system/webasha-heartbeat.service (ExecStart=/usr/bin/sleep infinity, WantedBy=multi-user.target), reload systemd, and start + enable webasha-heartbeat.