Since we're publishing documentation to docs.opendev.org now, drop
the openstackdocstheme extension and just use the default Alabaster
theme instead. Also clean up references to OpenStack in the Sphinx
configuration and remove a really old (and unnecessary) copyright
year.
Change-Id: I535783db9dccd36a094d41e6cf08469b6d8c3894
d3afb8039f