Add namespace to _list_opts() in test

After https://review.openstack.org/#/c/334014/, loading
NamedExtensionManager will fail when names is not an
iterable. In test test_list_ignores_doubles we are passing
None as namespace so it's failing.

Change-Id: I98e083c88a9d3db90bfeb9d5019d85bf01db6ca8

oslo_config/tests/test_generator.py

@@ -983,7 +983,7 @@ class IgnoreDoublesTestCase(base.BaseTestCase):
         ]
 
         slurped_opts = 0
-        for _, listing in generator._list_opts(None):
+        for _, listing in generator._list_opts(['namespace']):
             for _, opts in listing:
                 slurped_opts += len(opts)
         self.assertEqual(2, slurped_opts)