Add specific docs build option to tox

Now the documentation can be build in a clean virtual env with "tox
-edocs".

Change-Id: I0f2752f200caa21ad0f26e53df5c2757dd94c78a

tox.ini

@@ -48,6 +48,9 @@ commands = {toxinidir}/tools/config/generate_sample.sh -b . -p cinder -o etc/cin
 [testenv:venv]
 commands = {posargs}
 
+[testenv:docs]
+commands = python setup.py build_sphinx
+
 [flake8]
 # Following checks are ignored on purpose.
 #