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;
cleanup for Python 3 only
- Fix sphinx warnings
Change-Id: Ic70971b3c927508cd8a69b74f0c6b152671e8f83
c4a651a302