mqtt: add basic reporter

This change adds a MQTT reporter to publish build results message.

Change-Id: I5a9937a7952beac5c77d83ab791d48ff000b447b

etc/zuul.conf-sample

@@ -56,6 +56,13 @@ port=25
 default_from=zuul@example.com
 default_to=you@example.com
 
+[connection mqtt]
+driver=mqtt
+server=localhost
+user=zuul
+password=zuul
+;keepalive=60
+
 [connection mydatabase]
 driver=sql
 dburi=mysql+pymysql://user@localhost/zuul