Allow plugins to override initial network creation

The same code is already in place for the new lib/neutron library, allow
this functionality to be used also when the neutron legacy services are
still being deployed. This was mangled in [0].

[0] I868afeb065d80d8ccd57630b90658e330ab94251

Change-Id: I7214c4893943fbfbeb42ad140f433eecd6c3e9f0
diff --git a/stack.sh b/stack.sh
index 2528e2b..446d894 100755
--- a/stack.sh
+++ b/stack.sh
@@ -1315,7 +1315,14 @@
 # Once neutron agents are started setup initial network elements
 if is_service_enabled q-svc && [[ "$NEUTRON_CREATE_INITIAL_NETWORKS" == "True" ]]; then
     echo_summary "Creating initial neutron network elements"
-    create_neutron_initial_network
+    # Here's where plugins can wire up their own networks instead
+    # of the code in lib/neutron_plugins/services/l3
+    if type -p neutron_plugin_create_initial_networks > /dev/null; then
+        neutron_plugin_create_initial_networks
+    else
+        create_neutron_initial_network
+    fi
+
 fi
 
 if is_service_enabled nova; then