Automated CI testing

[penguineer]

This role has become rather complex and now runs on different (also production) systems. I believe it is time to set up automated testing to ensure our changes don't break already tested behavior.

[penguineer]

Possible tasks:

• Basic test infrastructure: molecule test (which just passes) and CI integration
• Verify APT repo setup
• Verify Docker installation
• Verify Prune setup
• Verify Prune behavior

[penguineer]

I have done automated testing for ansible roles (including CRON behavior) before in this repo: https://github.com/penguineer/ansible-role-dehydrated_cron

[LeSpocky]

FTR: This is not testing, but #46 introduces a workflow running ansible-lint (which also runs yamllint).