Draft: Distributed caching on GitLab runners

As discussed on IRC, The GitLab CI runners have a bug which causes file cache to be a bit unreliable. A solution to this is to configure distributed cache, for which MinIO can be used as an S3-compatible object store. The actual runner configuration would, if this MR is accepted, be done in a separate MR.

The minio role is pretty transparent when it comes to configuration, so if a multi-server MinIO setup is desired for example, it shouldn't be a problem.

Basic testing is set up using molecule with the vagrant driver and the testinfra verifer. This could be set up in GitLab CI in the future. Although the podman or docker drivers might be more suitable in that case. The reason it doesn't use any of those already is because I had trouble getting systemd working correctly with these drivers.