Stop Neutron advanced service external processes

Some of Neutron advanced services (LBaaS, VPNaaS, ....) creates
external processes and they should be stopped in unstack.
This commit defines neutron_<XaaS>_stop functions for all services
and implements the cleanup logics if necessary.

Also cleanup_neutron removes netns used by LBaaS haproxy.

Change-Id: Ied3a2c374ffcb6b59ecaf1027fb6e6083eded2ae
diff --git a/lib/neutron_plugins/services/metering b/lib/neutron_plugins/services/metering
index 629f3b7..b105429 100644
--- a/lib/neutron_plugins/services/metering
+++ b/lib/neutron_plugins/services/metering
@@ -26,5 +26,9 @@
     cp $NEUTRON_DIR/etc/metering_agent.ini $METERING_AGENT_CONF_FILENAME
 }
 
+function neutron_metering_stop() {
+    :
+}
+
 # Restore xtrace
 $MY_XTRACE