Use Producer.auto_declare to declare exchanges as needed

Connection has a cache of which entities (exchange/queue)
have been declared, so this shouldn't have too much of a
performance impact. This does, however, make tests much more
reliable.

st2common/st2common/transport/publishers.py

@@ -71,7 +71,9 @@ def do_publish(connection, channel):
                     # completely invalidating this ConnectionPool. Also, a ConnectionPool for
                     # producer does not really solve any problems for us so better to create a
                     # Producer for each publish.
-                    producer = Producer(channel)
+                    # passing exchange to Producer __init__ allows auto_declare to declare
+                    # anything that's missing (especially useful for tests).
+                    producer = Producer(channel, exchange=exchange)
                     kwargs = {
                         "body": payload,
                         "exchange": exchange,