lib/neutron-legacy - Use stevedore alias for ML2 plugin

Similar to 30ab23cd9b103470a7d89c4c88bccba789884c36, fix the
plugin name to avoid warnings like:

WARNING stevedore.named [-] Could not load neutron.plugins.ml2.plugin.Ml2Plugin

Change-Id: Ibb45f1305816b255ba2419ba662d9e29eff68f58
diff --git a/lib/neutron_plugins/ml2 b/lib/neutron_plugins/ml2
index e429714..eed0fc2 100644
--- a/lib/neutron_plugins/ml2
+++ b/lib/neutron_plugins/ml2
@@ -63,7 +63,7 @@
 function neutron_plugin_configure_common {
     Q_PLUGIN_CONF_PATH=etc/neutron/plugins/ml2
     Q_PLUGIN_CONF_FILENAME=ml2_conf.ini
-    Q_PLUGIN_CLASS="neutron.plugins.ml2.plugin.Ml2Plugin"
+    Q_PLUGIN_CLASS="ml2"
     # The ML2 plugin delegates L3 routing/NAT functionality to
     # the L3 service plugin which must therefore be specified.
     _neutron_service_plugin_class_add $ML2_L3_PLUGIN