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.
kind of gross to import in the actual call to network_create, but @gsilvis sez importing at startup messes up the test suites. At some point we'll need to handle multiple switches, which means that we need to handle switches of different types, but this will require some re-writing in a number of areas including this.