Switch to openstackdocstheme 2.1.2 and reno 3.1.0 versions. Using
these versions will allow parallelizing building of documents.
Update Sphinx version as well.
openstackdocstheme renames some variables, so follow the renames. A
couple of variables are also not needed anymore, remove them.
Set openstackdocs_auto_version to not version the documents.
Set openstackdocs_auto_name to use project as name.
Depends-On: https://review.opendev.org/728432
Change-Id: I4e3ae3ceabe125ea459ed4baabf2e98686268e50
6c963e7712