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/trove b/lib/trove
index f8e3edd..870afbe 100644
--- a/lib/trove
+++ b/lib/trove
@@ -198,7 +198,7 @@
 function stop_trove() {
     # Kill the trove screen windows
     for serv in tr-api tr-tmgr tr-cond; do
-        screen -S $SCREEN_NAME -p $serv -X kill
+        screen_stop $serv
     done
 }