**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
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_tree_subdag(self):
        url = 'tree?dag_id=example_subdag_operator.section-1'
        resp = self.client.get(url, follow_redirects=True)
        self.check_content_in_response('section-1-task-1', resp)
    def test_duration(self):
        url = 'duration?days=30&dag_id=example_bash_operator'
        resp = self.client.get(url, follow_redirects=True)
        self.check_content_in_response('example_bash_operator', resp)
    def test_duration_missing(self):
        url = 'duration?days=30&dag_id=missing_dag'
        resp = self.client.get(url, follow_redirects=True)
        self.check_content_in_response('seems to be missing', resp)
    def test_tries(self):