Add venv tox environment

This environment is used by the normal docs job, add it.
Manually add requirements needed for doc building.

Change-Id: I1be193d113683966f6a76e862713f3a550543168

Author: ajaeger

tox.ini

@@ -49,3 +49,13 @@ setenv =
   TOP_DIR={toxinidir}
 commands =
   python setup.py build_sphinx
+
+[testenv:venv]
+deps =
+   pbr>=0.6,!=0.7,<1.0
+   sphinx>=1.1.2,<1.2
+   oslosphinx
+   blockdiag
+   sphinxcontrib-blockdiag
+   sphinxcontrib-nwdiag
+commands = {posargs}