Tolerate missing openstackdocstheme in docs

Change-Id: I658d08fdb0e9946a660b079571b406d935e34441

doc/source/conf.py

@@ -21,10 +21,14 @@ sys.path.insert(0, os.path.abspath('../..'))
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom ones.
 extensions = [
     'sphinx.ext.autodoc',
-    #'sphinx.ext.intersphinx',
-    'openstackdocstheme',
 ]
 
+try:
+    import openstackdocstheme
+    extensions.append('openstackdocstheme')
+except ImportError:
+    openstackdocstheme = None
+
 # autodoc generation is a bit aggressive and a nuisance when doing heavy
 # text edit cycles.
 # execute "export SPHINX_DEBUG=1" in your terminal to disable
@@ -56,7 +60,10 @@ pygments_style = 'sphinx'
 # html_theme_path = ["."]
 # html_theme = '_theme'
 # html_static_path = ['static']
-html_theme = 'openstackdocs'
+if openstackdocstheme is not None:
+    html_theme = 'openstackdocs'
+else:
+    html_theme = 'default'
 
 # Output file base name for HTML help builder.
 htmlhelp_basename = '%sdoc' % project