More VRF fixes

**Resolves issue #326.**

This pull request...
- makes the `vrf` field on the `Network` model a required field and sets the default to the "Legacy" VRF, which was added as a fixture
- gets rid of "ip-phones-hack" and "avaya-hack", because those are wrong
- removes the `ALLOW_LEGACY_AND_VRF` range allow option

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- [x] Tests pass
- [x] Maintain migration works
- [x] DHCP build process is unaltered except for the intended changes