Add venv tox environment

Some OpenStack CI jobs need the venv tox environment to run, add it.

Change-Id: I6f93b1195797a85c2874f69e30c980d3cbd4ea6e

tox.ini

@@ -14,6 +14,9 @@ commands =
   find . -type f -name "*.pyc" -delete
   rm -Rf .testrepository/times.dbm
 
+[testenv:venv]
+commands = {posargs}
+
 [testenv:docs]
 commands =
     rm -rf doc/build