**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.test.assertNotIn(f'name="base_date" value="{base_date}"', data)
        self.test.assertNotIn('<option selected="" value="{num}">{num}</option>'.format(num=num_runs), data)
    def assert_run_is_not_in_dropdown(self, run, data):
        self.test.assertNotIn(run.execution_date.isoformat(), data)
        self.test.assertNotIn(run.run_id, data)
    def assert_run_is_in_dropdown_not_selected(self, run, data):
        self.test.assertIn(f'<option value="{run.execution_date.isoformat()}">{run.run_id}</option>', data)
    def assert_run_is_selected(self, run, data):
        self.test.assertIn(
            f'<option selected value="{run.execution_date.isoformat()}">{run.run_id}</option>', data
        )
    def test_with_default_parameters(self):
        """
        Tests view with no URL parameter.
        Should show all dag runs in the drop down.
        Should select the latest dag run.
        Should set base date to current date (not asserted)
        """
        response = self.test.client.get(
            self.endpoint, data=dict(username='test', password='test'), follow_redirects=True
        )
        self.test.assertEqual(response.status_code, 200)
        data = response.data.decode('utf-8')
        self.test.assertIn('<label class="sr-only" for="base_date">Base date</label>', data)
        self.test.assertIn('<label class="sr-only" for="num_runs">Number of runs</label>', data)
        self.assert_run_is_selected(self.runs[0], data)
        self.assert_run_is_in_dropdown_not_selected(self.runs[1], data)
        self.assert_run_is_in_dropdown_not_selected(self.runs[2], data)
        self.assert_run_is_in_dropdown_not_selected(self.runs[3], data)
    def test_with_execution_date_parameter_only(self):