Create a test machine registration service

Just like we have done with issue #2 (closed), we should aim at auto discovery of the test machine's information (MAC address, ...) and auto configuration (machine name, tftp & dnsmasq, gitlab runner).

With such a system, the infra would not need any fixed configuration for the test machines (save the configuration for the powercutter which cannot be discovered), and thus make it easy to create multiple deployments of the valve-infra, without having to land code to add machines to the infra.

This would simplify testing of different configurations, ahead of production, and can some day be useful to create a decentralized test farm.