Robustify service shutdown

* Save PID when using screen in screen_it()
* Add screen_stop()
* Call out service stop_*() in unstack.sh functions so screen_stop()
  can do its thing

Closes-bug: 1183449
Change-Id: Iac84231cfda960c4197de5b6e8ba6eb19225169a
diff --git a/lib/nova b/lib/nova
index 39685a8..178f8ee 100644
--- a/lib/nova
+++ b/lib/nova
@@ -705,7 +705,7 @@
     # Some services are listed here twice since more than one instance
     # of a service may be running in certain configs.
     for serv in n-api n-cpu n-crt n-net n-sch n-novnc n-xvnc n-cauth n-spice n-cond n-cell n-cell n-api-meta; do
-        screen -S $SCREEN_NAME -p $serv -X kill
+        screen_stop $serv
     done
     if is_service_enabled n-cpu && [[ -r $NOVA_PLUGINS/hypervisor-$VIRT_DRIVER ]]; then
         stop_nova_hypervisor