**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",
            "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, dt(2018, 7, 6, 5, 4, 3, tzinfo=timezone.TIMEZONE))
    def test_create_dagrun_valid_conf(self):
        conf_value = dict(Valid=True)
        data = {
            "state": "running",
            "dag_id": "example_bash_operator",
            "execution_date": "2018-07-06 05:05:03-02:00",
            "run_id": "test_create_dagrun_valid_conf",
            "conf": json.dumps(conf_value),
        }
        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.conf, conf_value)
    def test_create_dagrun_invalid_conf(self):