Update tox.ini to also run nova tests

This updates testr.conf to allow passing of the tests directory as an
environment variable (defaulting to ironic/tests/). Also updates tox.ini
to run testr once for ironic/tests and once for ironic/nova/tests/, then
combine subunit output.

test-requirements.txt is updated to pull in Nova from a tarball, as well
as mox and python-ironicclient.

Updates the existing Nova driver unit tests to account for recent
refactoring and changes in the Nova virt API.

Change-Id: Ibefa2e0d3f97512f4da9ace780703fe47467232e
Closes-bug: #1299312
Merge "Update tox.ini to also run nova tests"