Add missing ml2, L2 and L3 agent functions to devstack

Previously those functions were defined in the neutron's devstack plugin
but with [1] we moved qos related code into devstack and we missed about
moving them too.
This is follow up patch to fix that issue.

[1] https://review.opendev.org/c/openstack/devstack/+/815686

Change-Id: Icf459a2f8c6ae3c3cb29b16ba0b92766af41af30
diff --git a/lib/neutron_plugins/ml2 b/lib/neutron_plugins/ml2
index e1f868f..f00feac 100644
--- a/lib/neutron_plugins/ml2
+++ b/lib/neutron_plugins/ml2
@@ -156,5 +156,9 @@
     return 0
 }
 
+function configure_qos_ml2 {
+    neutron_ml2_extension_driver_add "qos"
+}
+
 # Restore xtrace
 $_XTRACE_NEUTRON_ML2