Schema and migration fixes

This pull request does the following:

- Removes the `vrf` field on the `StaticInterface` and `DynamicInterface` models.
    - (Rationale: `StaticInterface` doesn't need a `vrf` field because it's only allowed to be at its fixed IP. `DynamicInterface` doesn't need a `vrf` field because it's an interface's membership in a range in a network in a VRF that determines the DHCP class it's put it.)
- Deletes all the South migrations on the `soa`, `static_intr`, and `dynamic_intr` apps.
    - (Rationale: Some of those migrations were broken in that they didn't freeze the ORM. An ORM freeze in each migration is necessary for automatic creation of schema migrations. *FURTHER MISTAKES IN THIS REGARD ARE FORBIDDEN*.)
- Recreates the initial migration for those same apps.

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***This pull request requires that you drop the database and then re-run South and Maintain migrations.*** Sorry, but it was the only way to do what had to be done.