Move openstackdocstheme to extensions in api-ref

Move openstackdocstheme to extensions. According to the guide below:
https://docs.openstack.org/openstackdocstheme/latest/

Change-Id: Ibfb75e2a484b09b18cd18a4b2d21d559d7d4b475

api-ref/source/conf.py

@@ -28,8 +28,6 @@ import os
 import subprocess
 import sys
 
-import openstackdocstheme  # noqa
-
 # If extensions (or modules to document with autodoc) are in another directory,
 # add these directories to sys.path here. If the directory is relative to the
 # documentation root, use os.path.abspath to make it absolute, like shown here.
@@ -43,7 +41,8 @@ sys.path.insert(0, os.path.abspath('./'))
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom ones.
 
 extensions = [
-    'os_api_ref'
+    'os_api_ref',
+    'openstackdocstheme',
 ]
 
 # The suffix of source filenames.
@@ -56,23 +55,12 @@ source_suffix = '.rst'
 # The master toctree document.
 master_doc = 'index'
 
-# General information about the project.
-project = u'Block Storage API Reference'
 copyright = u'OpenStack Foundation'
 
-# The version info for the project you're documenting, acts as replacement for
-# |version| and |release|, also used in various other places throughout the
-# built documents.
-#
-from cinder.version import version_info
-# The full version, including alpha/beta/rc tags.
-release = version_info.release_string()
-# The short X.Y version.
-version = version_info.version_string()
-
-# html_context allows us to pass arbitrary values into the html template
-html_context = {"bug_tag": "api-ref",
-                "bug_project": "cinder"}
+# openstackdocstheme options
+repository_name = 'openstack/cinder'
+bug_project = 'cinder'
+bug_tag = 'api-ref'
 
 # The language for content autogenerated by Sphinx. Refer to documentation
 # for a list of supported languages.
@@ -113,11 +101,9 @@ pygments_style = 'sphinx'
 
 # The theme to use for HTML and HTML Help pages.  Major themes that come with
 # Sphinx are currently 'default' and 'sphinxdoc'.
-# html_theme_path = ["."]
 # html_theme = '_theme'
 
 html_theme = 'openstackdocs'
-html_theme_path = [openstackdocstheme.get_html_theme_path()]
 html_theme_options = {
     "sidebar_mode": "toc",
 }