Switch to newer openstackdocstheme and reno versions

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

api-ref/source/conf.py

@@ -43,9 +43,9 @@ master_doc = 'index'
 copyright = u'2010-present, OpenStack Foundation'
 
 # openstackdocstheme options
-repository_name = 'openstack/nova'
-bug_project = 'nova'
-bug_tag = 'api-ref'
+openstackdocs_repo_name = 'openstack/nova'
+openstackdocs_bug_project = 'nova'
+openstackdocs_bug_tag = 'api-ref'
 
 # The name of the Pygments (syntax highlighting) style to use.
 pygments_style = 'sphinx'