Shut down ironic services in the modern way

Instead of killing the screen, use stop_process which will
shut the processes when USE_SCREEN is False.

Change-Id: If0f714cb112dbf5fe9e4fdd7291cb4fb1df87f42
diff --git a/lib/ironic b/lib/ironic
index 0d7c127..35b5411 100644
--- a/lib/ironic
+++ b/lib/ironic
@@ -482,9 +482,8 @@
 
 # stop_ironic() - Stop running processes
 function stop_ironic {
-    # Kill the Ironic screen windows
-    screen -S $SCREEN_NAME -p ir-api -X kill
-    screen -S $SCREEN_NAME -p ir-cond -X kill
+    stop_process ir-api
+    stop_process ir-cond
 
     # Cleanup the WSGI files
     if [[ "$IRONIC_IPXE_ENABLED" == "True" ]] ; then