trivial: have tox install vms and nova into venv

Instead of using nova and vms python code in-tree by setting PYTHONPATH, this
patch makes tox install vms and nova into the virtual environment. Testing
with the installed python code better simulates a real deployment.

You can still run tests against in-tree nova, vms, and cobalt code by using the
py27-dev and py26-dev virtual environments.

Change-Id: I42a32e516a881beb64e24d514eade4ef7fde31e5
Merge "trivial: have tox install vms and nova into venv" into next/grizzly