Enhance recommendations for GitLab CI runners, and offer ready-to-run Ansible definitions

We have some instructions about how to set up customer GitLab CI runners in RUNNERS.md. These instructions are however very short, and there is plenty of things that could be done to harden security of runners and to improve the performance and efficiency of them.

We should research what is the truly most optimal way to set up a runner, and provide Ansible definitions for it so it is as easy as possible for more people to provide customer GitLab CI runners that run Salsa CI reliably and fast.