Expose our vendor interface on TeethDriver

We have a vendor interface already, but it isn't registered on
TeethDriver. This also renames 'teeth_ironic_driver.passthru' to
'teeth_ironic_driver.vendor', and TeethVendorPassthru to
TeethVendorInterface.
Merge pull request #16 from rackerlabs/use_vendor_interface

Expose our vendor interface on TeethDriver