Collect some monitoring host details automatically.

Fixes 80.

In working on #84, I noticed the TBD: derive these automatically comments about defining the hosts to be monitored. Since I'd need to add to these lists, I took a bit of time to see if I could make these values be derived automatically instead. I also took a bit of time to remove the remaining customize-*.nix file. (These two changes are separate commits, so you can review them separately if desired)

I think in an ideal world, I think this logic would live in some nixos/modules module, but since these values are needed by both the VPN and server modules which don't currently know about each other, it was easier to put it in morph/lib for the moment.

This MR generates the exact same derivations for all the systems both before and after, so is a pure refactoring.