**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.create_user_and_login(
            username='tree_success_for_read_only_role_user',
            role_name='tree_success_for_read_only_role_role',
            perms=[
                (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG),
                (permissions.ACTION_CAN_READ, permissions.RESOURCE_TASK_INSTANCE),
                (permissions.ACTION_CAN_READ, permissions.RESOURCE_TASK_LOG),
                (permissions.ACTION_CAN_READ, permissions.RESOURCE_WEBSITE),
            ],
        )
        url = 'tree?dag_id=example_bash_operator'
        resp = self.client.get(url, follow_redirects=True)
        self.check_content_in_response('runme_1', resp)
    def test_log_success(self):