Update docs building and do a few cleanups
* Switch to sphinx-build
* Update requirements for Sphinx and openstackdocstheme for
python 3, create doc/requirements.txt
* Remove unneeded doc and translation sections from setup.cfg
* Remove install_command, it's unneeded, the default is fine.
* Use TOX_CONSTRAINTS instead of obsolete UPPER_CONSTRAINTS
* Update hacking to 3.0
Change-Id: Ief34ac947ec23d6a9d4aad3640a4a61d0173578b