Import fixes from the Nova driver reviews

Import fixes from the Nova driver review, based upon revision 4
of https://review.openstack.org/#/c/111423/

- Add unittests for patcher.create()
- Convert some of the tests for the PXEDriverFields to GenericDriverFields
  and make sure that the delta from the PXE class is covered

Change-Id: Iec8d1d379458b89a2140dc473ad691dc5dd6d3dc
Merge "Import fixes from the Nova driver reviews"