Subnet types

This includes two things:
1) replace the initial subnet types list with a fuller list, and related refactoring
2) the first iteration of the NetworkQuery API/helper class for grabbing interfaces, etc for hosts in the context of a particular subnet type.

1) is largely done here; 2) will need more work, but that could be done either as part of this PR, or as a follow-on PR after merging this one, since it's not referenced anywhere yet.
This is how I do things elsewhere too (for roles, etc.). At the moment we don't support custom/manual additions to these records (there's no controller class, etc.), and the code assumes that things are exactly as created in seeds. As such, I think destroying, etc. is fair game here. Also, since we don't officially support upgrades in prod environments, real customers won't be re-running seeds.rb when there's real data, at least not yet.