Add venv testenv to tox.ini

This is required by the tox role run from python-tarball in the post ci job.

Change-Id: If85d0c902eada277bb95780d728a81e06fc9d86f

tox.ini

@@ -6,11 +6,17 @@ skipsdist = True
 [testenv]
 usedevelop = True
 install_command = pip install -U {opts} {packages}
+setenv =
+   VIRTUAL_ENV={envdir}
 deps =
     -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
     -r{toxinidir}/requirements.txt
     -r{toxinidir}/test-requirements.txt
 
+[testenv:venv]
+basepython = python3
+commands = {posargs}
+
 [testenv:pep8]
 basepython = python3
 commands = flake8