**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
get_default_executor_function.return_value = CeleryExecutor()
        task_id = 'runme_0'
        for state in QUEUEABLE_STATES:
            self.assertFalse(state in RUNNABLE_STATES)
            self.session.query(models.TaskInstance).filter(models.TaskInstance.task_id == task_id).update(
                {'state': state, 'end_date': timezone.utcnow()}
            )
            self.session.commit()
            form = dict(
                task_id=task_id,
                dag_id="example_bash_operator",
                ignore_all_deps="false",
                ignore_ti_state="false",
                execution_date=self.EXAMPLE_DAG_DEFAULT_DATE,
                origin='/home',
            )
            resp = self.client.post('run', data=form, follow_redirects=True)
            self.check_content_in_response('', resp)
            msg = (
                f"Task is in the &#39;{state}&#39; state which is not a valid state for execution. "
                + "The task must be cleared in order to be run"
            )
            self.assertTrue(re.search(msg, resp.get_data(as_text=True)))
    def test_refresh(self):