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/firewall b/lib/neutron_plugins/services/firewall
index 1597e85..580071f 100644
--- a/lib/neutron_plugins/services/firewall
+++ b/lib/neutron_plugins/services/firewall
@@ -23,5 +23,9 @@
     iniset_multiline $FWAAS_DRIVER_CONF_FILENAME fwaas driver "neutron.services.firewall.drivers.linux.iptables_fwaas.IptablesFwaasDriver"
 }
 
+function neutron_fwaas_stop() {
+    :
+}
+
 # Restore xtrace
 $MY_XTRACE