Add CI to test the playbook

[k3s-io]

Add CI to test the playbook

[geerlingguy]

I typically use GitHub Actions or Travis CI, so I'm most familiar with those.

For my Ansible playbook testing (which usually requires testing systemd services), I use Molecule and some Docker images that have systemd built-in; for example: https://github.com/geerlingguy/ansible-for-devops/tree/master/molecule

The one part that gets a little tricky is if we want to test a master and node, that would require two containers, and the networking can be a little tricky there.

[St0rmingBr4in]

Molecule seems the way to go indeed.

[fnord123]

One finding from #40 is that not only does the OS of the target machines (that will run k3s) matter, but that the OS of the Ansible host matters, which is obvious in retrospect. So a full featured CI environment should include not only the set of OS that are supported as target devices, but also the set of OS that are supported as Ansible host devices.