Add a tox job for generating docs

Add a tox job to make it easier for developers to generate their
own docs before submitting a patch.

Change-Id: I9c53ce664b164003fe42ed03d1f850d954bf0971

.gitignore

@@ -10,6 +10,7 @@ ChangeLog
 run_tests.err.log
 .tox
 doc/source/api
+doc/build
 *.egg
 heatclient/versioninfo
 *.egg-info

tox.ini

@@ -24,6 +24,10 @@ commands = {posargs}
 [testenv:cover]
 commands = python setup.py testr --coverage --testr-args='{posargs}'
 
+[testenv:docs]
+commands=
+    python setup.py build_sphinx
+
 [tox:jenkins]
 downloadcache = ~/cache/pip