**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
DAG_ID = 'dag_for_testing_dt_nr_dr_form'
    DEFAULT_DATE = datetime(2017, 9, 1)
    RUNS_DATA = [
        ('dag_run_for_testing_dt_nr_dr_form_4', datetime(2018, 4, 4)),
        ('dag_run_for_testing_dt_nr_dr_form_3', datetime(2018, 3, 3)),
        ('dag_run_for_testing_dt_nr_dr_form_2', datetime(2018, 2, 2)),
        ('dag_run_for_testing_dt_nr_dr_form_1', datetime(2018, 1, 1)),
    ]
    def __init__(self, test, endpoint):
        self.test = test
        self.endpoint = endpoint
        self.runs = []
    def setup(self):
        dagbag = self.test.app.dag_bag
        dag = DAG(self.DAG_ID, start_date=self.DEFAULT_DATE)
        dagbag.bag_dag(dag=dag, root_dag=dag)
        for run_data in self.RUNS_DATA:
            run = dag.create_dagrun(
                run_id=run_data[0], execution_date=run_data[1], state=State.SUCCESS, external_trigger=True
            )
            self.runs.append(run)
    def teardown(self):
        self.test.session.query(DagRun).filter(DagRun.dag_id == self.DAG_ID).delete()
        self.test.session.commit()
        self.test.session.close()
    def assert_base_date_and_num_runs(self, base_date, num_runs, data):