Add clean.sh

clean.sh gets rid of all residue of running DevStack except installed
packages and pip modules.

And it eradicates rabbitmq-server and ts erlang dependencies as well as
the other RPC backends and databases.

Change-Id: I2b9a251a0a151c012bae85a5a2f9c2f72e7700be
diff --git a/stack.sh b/stack.sh
index 7b4b3b7..14bb161 100755
--- a/stack.sh
+++ b/stack.sh
@@ -725,6 +725,8 @@
 configure_glanceclient
 
 if is_service_enabled nova; then
+    # First clean up old instances
+    cleanup_nova
     configure_nova
 fi
 if is_service_enabled horizon; then