neutron-legacy: Defer service_plugins configuration

This allows post-config phase to use neutron_service_plugin_class_add
so that devstack plugins can use it in the same way for both of
neutron-legacy and its "modern" counterpart, lib/neutron.

Closes-Bug: #1667037
Change-Id: I9068fd608e82e70db8d725f92269a26920efebcb
diff --git a/stack.sh b/stack.sh
index 7d440a7..19b07ab 100755
--- a/stack.sh
+++ b/stack.sh
@@ -1241,6 +1241,7 @@
     start_neutron_api
 elif is_service_enabled q-svc; then
     echo_summary "Starting Neutron"
+    configure_neutron_after_post_config
     start_neutron_service_and_check
 elif is_service_enabled $DATABASE_BACKENDS && is_service_enabled n-net; then
     NM_CONF=${NOVA_CONF}