Code review comment for lp:~allenap/maas/ntp-prefer-nearby-servers--bug-1675095

Where we have no prior knowledge of which racks a machine has booted from — i.e. during enlistment — we configure NTP to talk to all _rack_ controllers. One address per rack is chosen according to the new metric calculation that this branch introduces. Only rack controllers talk NTP to region controllers, never machines or devices.

Thanks for testing this out. I'll give it a run through Jenkins anyway before I land.