Refactor Ironic driver tests as per review comment

During the Ironic driver merge into Nova, there were some
suggestions on refactoring the tests as per this review
(https://review.openstack.org/#/c/111425/19/nova/...
...tests/virt/ironic/test_driver.py)

This patch implements these suggestions.

Change-Id: I05e8b3d01bffebe2f68bbd50fc775e1ef48616cc
Merge "Refactor Ironic driver tests as per review comment"