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/cinder b/lib/cinder
index cbe732e..11414be 100644
--- a/lib/cinder
+++ b/lib/cinder
@@ -556,7 +556,7 @@
 function stop_cinder() {
     # Kill the cinder screen windows
     for serv in c-api c-bak c-sch c-vol; do
-        screen -S $SCREEN_NAME -p $serv -X kill
+        screen_stop $serv
     done
 
     if is_service_enabled c-vol; then