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/heat b/lib/heat
index e44a618..29cd967 100644
--- a/lib/heat
+++ b/lib/heat
@@ -175,7 +175,7 @@
 function stop_heat() {
     # Kill the screen windows
     for serv in h-eng h-api h-api-cfn h-api-cw; do
-        screen -S $SCREEN_NAME -p $serv -X kill
+        screen_stop $serv
     done
 }