Rehome functions to enable Neutron's Trunk service plugin

Those functions were part of the neutron devstack plugin but we
discussed on the neutron team meeting [1] to move it to the Devstack
repo as it's mature enough now.

[1] https://meetings.opendev.org/meetings/networking/2021/networking.2021-10-05-14.00.log.html#l-156

Change-Id: I35446adad1d8a7fed142d834de20c48b611015a5

lib/neutron-legacy

@@ -275,6 +275,10 @@ source $TOP_DIR/lib/neutron_plugins/services/metering
 
 # L3 Service functions
 source $TOP_DIR/lib/neutron_plugins/services/l3
+
+# Additional Neutron service plugins
+source $TOP_DIR/lib/neutron_plugins/services/trunk
+
 # Use security group or not
 if has_neutron_plugin_security_group; then
     Q_USE_SECGROUP=${Q_USE_SECGROUP:-True}
@@ -369,6 +373,11 @@ function configure_mutnauq {
         configure_ovn_plugin
     fi
 
+    # Configure Neutron's advanced services
+    if is_service_enabled q-trunk neutron-trunk; then
+        configure_trunk_extension
+    fi
+
     iniset $NEUTRON_CONF DEFAULT api_workers "$API_WORKERS"
     # devstack is not a tool for running uber scale OpenStack
     # clouds, therefore running without a dedicated RPC worker

lib/neutron_plugins/services/trunk

@@ -0,0 +1,5 @@
+#!/bin/bash
+
+function configure_trunk_extension {
+    neutron_service_plugin_class_add "trunk"
+}