**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 05:04:03-08:00",
            "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, 13, 4, 3))
    @conf_vars({("core", "default_timezone"): "America/Toronto"})
    def test_create_dagrun_execution_date_without_timezone_default_edt(self):
        data = {
            "state": "running",
            "dag_id": "example_bash_operator",
            "execution_date": "2018-07-06 05:04:03",
            "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_without_timezone_default_utc(self):