**Description**
In the trigger page, there is a text area to enter (optional) conf json. It would be great if a sample JSON can be provided programatically while defining DAG

**Use case / motivation**

This will improve usability of the UI that triggers a DAG

tests/www/test_views.py
data = {
            "state": "running",
            "dag_id": "example_bash_operator",
            "execution_date": "2018-07-06 05:06:03",
            "run_id": "test_create_dagrun_invalid_conf",
            "conf": "INVALID: [JSON",
        }
        resp = self.client.post('/dagrun/add', data=data, follow_redirects=True)
        self.check_content_in_response('JSON Validation Error:', resp)
        dr = self.session.query(models.DagRun).all()
        self.assertFalse(dr)
    def test_list_dagrun_includes_conf(self):
        data = {
            "state": "running",
            "dag_id": "example_bash_operator",
            "execution_date": "2018-07-06 05:06:03",
            "run_id": "test_list_dagrun_includes_conf",
            "conf": '{"include": "me"}',
        }
        self.client.post('/dagrun/add', data=data, follow_redirects=True)
        dr = self.session.query(models.DagRun).one()
        self.assertEqual(dr.execution_date, timezone.convert_to_utc(datetime(2018, 7, 6, 5, 6, 3)))
        self.assertEqual(dr.conf, {"include": "me"})
        resp = self.client.get('/dagrun/list', follow_redirects=True)
        self.check_content_in_response("{&#34;include&#34;: &#34;me&#34;}", resp)
    def test_clear_dag_runs_action(self):