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.
Remove duplicated variables.
Depends-On: https://review.opendev.org/728432
Change-Id: Icb604b31150a1ad6c29a9a4a934ed13f8a0b4976
553040ed4b