Show maintenance field in GET /nodes

Before the maintenance field was only shown if client issued a GET on
a specific node (node-show), but this is an important attribute to have
listed in a GET /nodes (node-list), it's a good info to know which nodes
from that list are in maintenance mode or not, and the Nova Ironic Driver
also need this information to determine which nodes are available to be
exposed to Nova or not.

Change-Id: Icc31a312130a1c2d5853da64bd9091a93221363c
Add Node.instance_info field

This is necessary for instance-level info accessed by drivers.

Related to blueprint add-node-instance-info

Change-Id: I4688676767a80ac6f1f3d571f0df4ea78bc756e1
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 "Add Node.instance_info field"
Merge "Show maintenance field in GET /nodes"
Merge "Update tox.ini to also run nova tests"