Use [] where a value is required

The use of .get() implies that having no value, or None is ok,
but in the case that's fixed here, a value of None would result
in an error, so [] is correct to use here.

Change-Id: I47113121c10c2d0682daf40eb616b379e2d212e8

keystone/auth/controllers.py

@@ -65,7 +65,7 @@ def load_auth_methods():
             plugin_name = driver.method
         else:
             plugin_name = plugin
-            plugin_class = CONF.auth.get(plugin)
+            plugin_class = CONF.auth[plugin]
             driver = importutils.import_object(plugin_class)
         if plugin_name in AUTH_METHODS:
             raise ValueError(_('Auth plugin %(plugin)s is requesting '