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
diff --git a/lib/rpc_backend b/lib/rpc_backend
index 981b80b..ec821f1 100644
--- a/lib/rpc_backend
+++ b/lib/rpc_backend
@@ -74,7 +74,8 @@
     if is_service_enabled rabbit; then
         # Obliterate rabbitmq-server
         uninstall_package rabbitmq-server
-        sudo killall epmd || sudo killall -9 epmd
+        # in case it's not actually running, /bin/true at the end
+        sudo killall epmd || sudo killall -9 epmd || /bin/true
         if is_ubuntu; then
             # And the Erlang runtime too
             apt_get purge -y erlang*