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.
@okreig, @zenhack and I have decided to pass in the database session here (and to the freeing function too).