This repo is now testing docs only with Python 3, so let's make
a few cleanups:
- Remove obsolete sections from setup.cfg
- Switch to using sphinx-build
- Cleanup doc/source/conf.py to remove now obsolete content.
- Use newer openstackdocstheme version
- Remove install_command from tox.ini, the default is fine
Change-Id: Iffd00261e6a15e2dd7c98e96bbcc9db3e86c0c65
34d54f2812