**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
self.clear_table(models.DagRun)
        self.clear_table(models.TaskInstance)
    def test_create_dagrun_execution_date_with_timezone_utc(self):
        data = {
            "state": "running",
            "dag_id": "example_bash_operator",
            "execution_date": "2018-07-06 05:04:03Z",
            "run_id": "test_create_dagrun",
        }
        resp = self.client.post('/dagrun/add', data=data, follow_redirects=True)
        self.check_content_in_response('Added Row', resp)
        dr = self.session.query(models.DagRun).one()
        self.assertEqual(dr.execution_date, timezone.datetime(2018, 7, 6, 5, 4, 3))
    def test_create_dagrun_execution_date_with_timezone_edt(self):
        data = {
            "state": "running",
            "dag_id": "example_bash_operator",
            "execution_date": "2018-07-06 05:04:03-04:00",
            "run_id": "test_create_dagrun",
        }
        resp = self.client.post('/dagrun/add', data=data, follow_redirects=True)
        self.check_content_in_response('Added Row', resp)
        dr = self.session.query(models.DagRun).one()
        self.assertEqual(dr.execution_date, timezone.datetime(2018, 7, 6, 9, 4, 3))
    def test_create_dagrun_execution_date_with_timezone_pst(self):