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/glance b/lib/glance
index 135136d..80868ae 100644
--- a/lib/glance
+++ b/lib/glance
@@ -206,8 +206,8 @@
 # stop_glance() - Stop running processes
 function stop_glance() {
     # Kill the Glance screen windows
-    screen -S $SCREEN_NAME -p g-api -X kill
-    screen -S $SCREEN_NAME -p g-reg -X kill
+    screen_stop g-api
+    screen_stop g-reg
 }