Establish driver API

See my first commit message for details.

This commit anticipates our openflow/opendaylight driver, which we should be able to create pretty soon.

Things that are needed:  Tests for the Dell driver---we need to test the logic of the vlan allocation functions, as well as eventual config parsing.
`9004` isn't actually valid. We aren't really validating any of the config yet (there's been a pull request out for that forever...) but if anything this *shouldn't* pass. just pick a lower number.