**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()
        form = dict(
            task_id="runme_0",
            dag_id="example_bash_operator",
            ignore_all_deps="false",
            ignore_ti_state="true",
            execution_date=self.default_date,
        )
        resp = self.client.post('run', data=form)
        self.check_content_in_response('', resp, resp_code=302)
    def test_run_success_for_all_dag_user(self):
        self.logout()
        self.login(username='all_dag_user', password='all_dag_user')
        form = dict(
            task_id="runme_0",
            dag_id="example_bash_operator",
            ignore_all_deps="false",
            ignore_ti_state="true",
            execution_date=self.default_date,
        )
        resp = self.client.post('run', data=form)
        self.check_content_in_response('', resp, resp_code=302)
    def test_blocked_success(self):