Add venv section in tox.ini

As jenkins docs job use venv envrionment to work, we
need add such section to make it work.

Partially-Implements: blueprint kolla-tox-doc
Change-Id: I716219515f2bf1375fd094b3aeca866cbcf5c7c6

tox.ini

@@ -12,6 +12,9 @@ deps = -r{toxinidir}/requirements.txt
 commands =
     flake8
 
+[testenv:venv]
+commands = {posargs}
+
 [testenv:docs]
 commands = python setup.py build_sphinx