trivial: private tox venv for cobalt branch

Since pip-requires and test-requires change between OpenStack
releases, tox can't use the same venv for different cobalt branches.
Thus when you switch between cobalt branches and run tests using tox,
tox has to rebuild the venv.

Change-Id: I2a42688709619f137a6290781a4ae273b2ed530a
Merge 'trivial: private tox venv for cobalt branch' from grizzly/1.5

Change-Id: Id25fd83bb8b34e35b7ddaa88332c367d41dc0c45