Fixes #7456 - Move interface-related attributes from the Host model to the Nic model

See https://groups.google.com/forum/#!topic/foreman-dev/ll3pZXrYeF4 for the discussion of the current state, and testing notes/gotchas.

I expect this is probably going to need extensive review, but that eventually (after we're all agreed on the approach at least) we'll have to bite the bullet and merge it (much as we did for the auth/permissions update). As before, we can roll-forward and fix any missed edge cases.

There are a number of areas this can be further improved, but we're open to doing that in separate PRs to at least *try* to keep this manageable.
Changed both to be statements about the purpose of the primary and provision interfaces, rather than non-intuitive questions.