add gating up/down script for devstack

This adds the test infrastructure for testing that unstack.sh and
clean.sh do the right thing, and actually stop what's expected. This
is designed to be used in upstream testing to make unstack and clean a
bit more certain.

It includes numerous fixes to make these pass in an errexit
environment with the gate config. The scripts still don't run under
errexit because we don't assume we've handled all possible cleanup safely.

Change-Id: I774dfb2cc934367eef2bb7ea5123197f6da7565b

lib/dstat

@@ -40,7 +40,10 @@ function start_dstat {
 
 # stop_dstat() stop dstat process
 function stop_dstat {
-    screen_stop dstat
+    # dstat runs as a console, not as a service, and isn't trackable
+    # via the normal mechanisms for devstack. So lets just do a
+    # killall and move on.
+    killall dstat || /bin/true
 }
 
 # Restore xtrace