Fix ResourceActionNotSupported errors in log

In 37ef66970510972af54ce6835ec234f3e30a0a88 (in Kilo) we made signalling
a resource asynchronous because things like e.g. scaling an autoscaling
group were too slow to do synchronously (bug 1394095).

However, for resources that don't have a handle_signal() method at all,
this means that if a signal other than one handled by Heat (e.g. to
clear a hook) is received then the user won't be notified of the error.
There will instead be an error message in the logs warning of an
unhandled exception in a thread.

(Previously, similar problems occurred if the user was attempting to
clear a hook and some error occurred. This has since been fixed as bug
1472515, in Liberty.)

If there's no handle_signal() method defined, then calling
Resource.signal() is quick (i.e. it's only going to clear a hook, or
raise an error). So handle this case synchronously. This ensures that
any error is reported to the user and not in the log.

Change-Id: I917b4c7d4ab2cde46148197a3a232d5dc70f63d7
Related-Bug: #1472515

api/gabbits/resources.yaml

@@ -82,7 +82,7 @@ tests:
 
 - name: signal resource
   POST: $LAST_URL/signal
-  status: 200
+  status: 400
 
 - name: delete stack with resources
   DELETE: /stacks/$ENVIRON['PREFIX']-rsrcstack