Distinguish between name not provided and incorrect

When loading from config we need a way to determine if a plugin name was
specified incorrectly or was not specified at all. We need this to
determine if we need to load a fallback plugin.

This is much more in line with how CLI loading works and how it should
have worked initially.

Change-Id: I5547b6e169abc4f1850ff205a8f054a617785c2c
Closes-Bug: #1359618

keystoneclient/auth/conf.py

@@ -13,7 +13,6 @@
 from oslo.config import cfg
 
 from keystoneclient.auth import base
-from keystoneclient import exceptions
 
 _AUTH_PLUGIN_OPT = cfg.StrOpt('auth_plugin', help='Name of the plugin to load')
 
@@ -89,7 +88,7 @@ def load_from_conf_options(conf, group, **kwargs):
     :param conf: An oslo.config conf object.
     :param string group: The group name that options should be read from.
 
-    :returns plugin: An authentication Plugin.
+    :returns plugin: An authentication Plugin or None if a name is not provided
 
     :raises exceptions.NoMatchingPlugin: if a plugin cannot be created.
     """
@@ -101,7 +100,7 @@ def load_from_conf_options(conf, group, **kwargs):
 
     name = conf[group].auth_plugin
     if not name:
-        raise exceptions.NoMatchingPlugin('No plugin name provided for config')
+        return None
 
     plugin_class = base.get_plugin_class(name)
     plugin_class.register_conf_options(conf, group)

keystoneclient/tests/auth/test_conf.py

@@ -101,10 +101,8 @@ class ConfTests(utils.TestCase):
                           self.GROUP)
 
     def test_loading_with_no_data(self):
-        self.assertRaises(exceptions.NoMatchingPlugin,
-                          conf.load_from_conf_options,
-                          self.conf_fixture.conf,
-                          self.GROUP)
+        self.assertIsNone(conf.load_from_conf_options(self.conf_fixture.conf,
+                                                      self.GROUP))
 
     @mock.patch('stevedore.DriverManager')
     def test_other_params(self, m):