Switch from oslosphinx to openstackdocstheme

To use openstackdocstheme 1.11.0 properly, this patch fixes
some settings according to follow.
https://docs.openstack.org/openstackdocstheme/latest/

Change-Id: I07652e3a086a650ab9f08f6da7ddf0e8145b2969
diff --git a/releasenotes/source/conf.py b/releasenotes/source/conf.py
index d240467..3137541 100644
--- a/releasenotes/source/conf.py
+++ b/releasenotes/source/conf.py
@@ -37,10 +37,18 @@
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
 # ones.
 extensions = [
-    'oslosphinx',
+    'openstackdocstheme',
     'reno.sphinxext',
 ]
 
+# openstackdocstheme options
+repository_name = 'openstack/tempest'
+bug_project = 'tempest'
+bug_tag = ''
+
+# Must set this variable to include year, month, day, hours, and minutes.
+html_last_updated_fmt = '%Y-%m-%d %H:%M'
+
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']
 
@@ -119,9 +127,6 @@
 # html_theme_options = {}
 
 # Add any paths that contain custom themes here, relative to this directory.
-import openstackdocstheme
-
-html_theme_path = [openstackdocstheme.get_html_theme_path()]
 
 # The name for this set of Sphinx documents.  If None, it defaults to
 # "<project> v<release> documentation".