Add a docs virtualenv

The various projects are standardizing on a docs venv which
will eventually be called by the infra project to build the
docs for a project.

To enable our usage of this environment add a section so that
it can be called upon.

Change-Id: I420eb0d6e6f9f6f24bc493d4342478daba27ad1e

tox.ini

@@ -2,6 +2,7 @@
 minversion = 1.6
 skipsdist = True
 envlist = cover,
+    docs,
     pep8,
     py26,
     py26-sa7-mysql,
@@ -21,6 +22,15 @@ deps = -r{toxinidir}/test-requirements.txt
     kombu>=2.4.8
 commands = python setup.py testr --slowest --testr-args='{posargs}'
 
+[testenv:docs]
+basepython = python2.7
+deps = -r{toxinidir}/requirements-py2.txt
+    -r{toxinidir}/test-requirements.txt
+    -r{toxinidir}/optional-requirements.txt
+    doc8
+commands = python setup.py build_sphinx
+    doc8 doc/source
+
 [tox:jenkins]
 downloadcache = ~/cache/pip