Record handler tasks in json job output

There's a separate callback method for handler tasks.  This is what
we do in the text stream callback plugin; we should do the same in
the json output.

Change-Id: I48273ec182032f198b8886be95b0cba5c6f4843e

zuul/ansible/base/callback/zuul_json.py

@@ -128,6 +128,9 @@ class CallbackModule(CallbackBase):
     def v2_playbook_on_task_start(self, task, is_conditional):
         self.results[-1]['tasks'].append(self._new_task(task))
 
+    def v2_playbook_on_handler_task_start(self, task):
+        self.v2_playbook_on_task_start(task, False)
+
     def v2_runner_on_ok(self, result, **kwargs):
         host = result._host
         action = result._task.action