Add docs tox env

Just like in nova, lets add a 'docs' env to here so you can simply run
'tox -edocs' and build the docs which get published to

http://docs.openstack.org/developer/python-novaclient/

Change-Id: Ie381a78477b60b4b9981576d75c911cb0deb5696

tox.ini

@@ -1,6 +1,6 @@
 # noted to use py34 you need virtualenv >= 1.11.4
 [tox]
-envlist = py26,py27,py33,py34,pypy,pep8
+envlist = py26,py27,py33,py34,pypy,pep8,docs
 minversion = 1.6
 skipsdist = True
 
@@ -26,6 +26,12 @@ commands = flake8 {posargs}
 [testenv:venv]
 commands = {posargs}
 
+[testenv:docs]
+commands =
+  python setup.py build_sphinx
+
+
+
 [testenv:functional]
 setenv =
   OS_TEST_PATH = ./novaclient/tests/functional