Refactor VLAN driver tests

New driver: null_vlan does nothing on network_apply, but allocates and frees
VLANs like the Dell driver does.  This makes unit tests a little nicer, I
think.
Refactor dell driver into simple_vlan driver

This driver works for any VLAN-based setup with a single switch.  It shells
out to the appropriate switch driver for any actual operations.
Update haas.cfg.example
Update deployment.cfg.example
Fix simple_vlan.py docstring
Test simple_vlan driver with a null switch
Fix three typos
Move 'trunk_port' option from 'switch dell' to 'driver simple_vlan'

This finally eliminates the 'switch dell' section, and will make it easier to
generalize the deployment tests
Factor Dell code out of deployment tests
Restore part of erroneously removed comment
Document limitations of deployment.cfg
Merge pull request #248 from gsilvis/driver-refactor

Driver refactor