Switch to newer openstackdocstheme and reno versions

Switch to openstackdocstheme 2.2.0 and reno 3.1.0 versions. Using
these versions will allow especially:
* Linking from HTML to PDF document
* parallelizing building of documents

Update Sphinx version as well.

openstackdocstheme renames some variables, so follow the renames
before the next release removes them. A couple of variables are also
not needed anymore, remove them.

Set openstackdocs_pdf_link to link to PDF file.

Depends-On: https://review.opendev.org/728938
Change-Id: Ia31f69f00eb24771bf700b4e25ae73a09c1c8aed
diff --git a/releasenotes/source/conf.py b/releasenotes/source/conf.py
index 92df4c4..71df749 100644
--- a/releasenotes/source/conf.py
+++ b/releasenotes/source/conf.py
@@ -42,9 +42,9 @@
 ]
 
 # openstackdocstheme options
-repository_name = 'openstack/tempest'
-bug_project = 'tempest'
-bug_tag = ''
+openstackdocs_repo_name = 'openstack/tempest'
+openstackdocs_bug_project = 'tempest'
+openstackdocs_bug_tag = ''
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']