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.
We need to be passing in the database connection here (and elsewhere...). Would you open a new pull request and address this? We should also do a second pass to make sure we didn't miss any other Very Important Things.