Add l3 conntrack helper to devstack plugin

This commit adds possibility to configure L3 conntrack_helper
service plugin and l3 extension with devstack plugin.

Related-Bug: #1823633
Change-Id: Ie96ff80f1c296c40ec2cd82c8d917a8bb262b12e

devstack/lib/l3_conntrack_helper

@@ -0,0 +1,4 @@
+function configure_l3_conntrack_helper {
+    neutron_service_plugin_class_add "conntrack_helper"
+    plugin_agent_add_l3_agent_extension "conntrack_helper"
+}

devstack/plugin.sh

@@ -5,6 +5,7 @@ source $LIBDIR/flavors
 source $LIBDIR/l2_agent
 source $LIBDIR/l2_agent_sriovnicswitch
 source $LIBDIR/l3_agent
+source $LIBDIR/l3_conntrack_helper
 source $LIBDIR/ml2
 source $LIBDIR/network_segment_range
 source $LIBDIR/qos
@@ -85,6 +86,9 @@ if [[ "$1" == "stack" ]]; then
                 if is_service_enabled q-port-forwarding neutron-port-forwarding; then
                     configure_port_forwarding
                 fi
+                if is_service_enabled q-conntrack-helper neutron-conntrack-helper; then
+                    configure_l3_conntrack_helper
+                fi
                 configure_l3_agent
             fi
             if [ $NEUTRON_CORE_PLUGIN = ml2 ]; then