builders: init profile based builder configuration

[mweinelt]

With the following ideas:

• Use ZFS for RAID0
• Use tmpfs for /tmp and host build directories in RAM
• Run all builds in cgroups
• Updates and reboots using system.autoUpgrade
• Network autoconfiguration

[mweinelt]

These are very much in production, so we can iterate on them further, but I'm merging them now, so we don't roll back any changes to hydras known_hosts or prometheus.