Set the basepython for the venv tox environment
Set the basepython for the venv tox environment

CI uses the venv tox environment to build the documentation. We need to
make sure that Python 2 is used until out docs build correctly under
Python 3.

Change-Id: I1a1d3123f22bf89965eb65e48648aae38cbdd254

openstack/mistral,openstack/mistral,StackStorm/mistral,StackStorm/mistral