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

tests/fixtures/config/job-output/git/common-config/playbooks/job-output.yaml

@@ -1,6 +1,10 @@
 - hosts: all
+  handlers:
+    - name: handler
+      shell: echo "This is a handler"
   tasks:
     - shell: echo "Standard output test {{ zuul.executor.src_root }}"
+      notify: handler
     - shell: echo "This should be skipped"
       when: false
     - fail:

tests/unit/test_v3.py

@@ -4926,6 +4926,10 @@ class TestJobOutput(AnsibleZuulTestCase):
                         ['hosts']['localhost']['skipped'])
         self.assertTrue(j[0]['plays'][0]['tasks'][2]
                         ['hosts']['localhost']['failed'])
+        self.assertEqual(
+            "This is a handler",
+            j[0]['plays'][0]['tasks'][3]
+            ['hosts']['localhost']['stdout'])
 
         self.log.info(self._get_file(self.history[0],
                                      'work/logs/job-output.txt'))

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