If the `certbot_auto_renew` variable was `True`, and the cronjob was
added, setting it to `False` would not remove it. By always including
the `renew-cron.yml` task file, and instead checking the
`certbot_auto_renew` value there, and pick a value for `state` based on
it, we can clean up a cronjob if it is no longer wanted.
- introduces a variable certbot_from_git
- adds install from package tasks
- makes a variable called certbot_script available (to handle certbot vs certbot-auto naming)
- fixes#18
this is a preparation for installing certbot from package management
additionally, it might be useful when further tasks are added, e.g. for initial certificate retrieval
