[Labs]: Switch to pull-based configuration

Deploying configuration changes to the labs has been a headache for years because we always have to worry if some computer is booted to Windows or is powered off.

Moving from a push-based approach to a pull-based approach solves this, as the labs will regularly pull the new configuration (for instance, every day at 04:00 AM) and labs that were turned off will pull the new configuration on boot. Now, what's committed to master will be the source of truth for every lab, no more forgotten labs with old configurations.

It may be useful to establish monitoring related to this, so as to be aware of broken services resulting from a configuration rebuild, and also being aware each computer's current configuration (for example, which commit's hash of this repo it's currently running).