Add item UUID to MQTT reporter

Since the MQTT reporter can be used to emit start or enqueue events,
it may be useful to match enqueue and dequeue events.  That could
be done with tenant+pipeline+change+patchset, but we also have a
UUID for queue items, so to make it simpler for MQTT consumers,
let's expose that.

Change-Id: Iff88bcfd73e00f292e0cc947f548582a276a7975

doc/source/reference/drivers/mqtt.rst

@@ -13,6 +13,12 @@ An MQTT report uses this schema:
 
 .. attr:: <mqtt schema>
 
+   .. attr:: uuid
+
+      The item UUID.  Each item enqueued into a Zuul pipeline is
+      assigned a UUID which remains the same even as Zuul's
+      speculative execution algorithm re-orders pipeline contents.
+
    .. attr:: action
 
       The reporter action name, e.g.: 'start', 'success', 'failure',

tests/unit/test_connection.py

@@ -652,6 +652,7 @@ class TestMQTTConnection(ZuulTestCase):
         self.assertIn('enqueue_time', mqtt_payload)
         self.assertIn('trigger_time', mqtt_payload)
         self.assertIn('zuul_event_id', mqtt_payload)
+        self.assertIn('uuid', mqtt_payload)
         self.assertEquals(dependent_test_job['dependencies'], ['test'])
 
     def test_mqtt_invalid_topic(self):

zuul/driver/mqtt/mqttreporter.py

@@ -48,6 +48,7 @@ class MQTTReporter(BaseReporter):
                 item, with_jobs=False),
             'trigger_time': item.event.timestamp,
             'enqueue_time': item.enqueue_time,
+            'uuid': item.uuid,
             'buildset': {
                 'uuid': item.current_build_set.uuid,
                 'result': item.current_build_set.result,