Update docs to use sphinx-build

Update the tox.ini helper env 'docs' to use sphinx-build directly like
the job does.

Use the build-sphinx-docs job rather than the openstack version as we do
not need constraints in zuul.

This does not move doc requirements to doc/requirements.txt because the
Zuul docs document the tests too, so the doc build needs all the things
in test-requirements as well.

Change-Id: Id81c05725d4dbe12dccc0272c396c92ff2f475ec
diff --git a/tox.ini b/tox.ini
index 5efc4c0..5326ba4 100644
--- a/tox.ini
+++ b/tox.ini
@@ -36,7 +36,8 @@
   python setup.py test --coverage
 
 [testenv:docs]
-commands = python setup.py build_sphinx
+commands =
+  sphinx-build -W -d doc/build/doctrees -b html doc/source/ doc/build/html
 
 [testenv:venv]
 commands = {posargs}