By running ansible-lint we check if playbooks provided in
ansible-freipa follow Ansible's best practices, nd the verification
will be performed on every push (even on forks) or pull-request.

This patch provides the configuration needed to run ansible-lint
to the playbooks found in the `tests`, `playbooks` and `molecule`
directories, on every push or pull-request done on Github, using
Ansible's Github Action ansible/ansible-lint-action.