**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.logout()
        self.login(username='dag_faker', password='dag_faker')
        url = 'log?task_id=runme_0&dag_id=example_bash_operator&execution_date={}'.format(
            self.percent_encode(self.default_date)
        )
        resp = self.client.get(url, follow_redirects=True)
        self.check_content_not_in_response('Log by attempts', resp)
        url = (
            'get_logs_with_metadata?task_id=runme_0&dag_id=example_bash_operator&'
            'execution_date={}&try_number=1&metadata=null'.format(self.percent_encode(self.default_date))
        )
        resp = self.client.get(url, follow_redirects=True)
        self.check_content_not_in_response('"message":', resp)
        self.check_content_not_in_response('"metadata":', resp)
    def test_log_success_for_user(self):
        self.logout()
        self.login(username='test_user', password='test_user')
        url = 'log?task_id=runme_0&dag_id=example_bash_operator&execution_date={}'.format(
            self.percent_encode(self.default_date)
        )
        resp = self.client.get(url, follow_redirects=True)
        self.check_content_in_response('Log by attempts', resp)
        url = (
            'get_logs_with_metadata?task_id=runme_0&dag_id=example_bash_operator&'
            'execution_date={}&try_number=1&metadata=null'.format(self.percent_encode(self.default_date))
        )
        resp = self.client.get(url, follow_redirects=True)
        self.check_content_in_response('"message":', resp)
        self.check_content_in_response('"metadata":', resp)
    def test_tree_view_for_viewer(self):