Declare parallel reading safe

Sphinx 1.3 or later allows extensions to declare parallel reading
or writing is safe or not [1]. parallel reading is disabled by default.
I believe we can enable this safely.

[1] http://www.sphinx-doc.org/en/stable/extdev/#extension-metadata

Change-Id: Icd4b2c9951bb7bda3217cc814985dcd26ec5b5ce

openstackdocstheme/__init__.py

@@ -113,3 +113,6 @@ def setup(app):
         'openstackdocs',
         os.path.abspath(os.path.dirname(__file__)) + '/theme/openstackdocs',
     )
+    return {
+        'parallel_read_safe': True,
+    }