**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='test', password='test')
        website_permission = self.appbuilder.sm.find_permission_view_menu(
            permissions.ACTION_CAN_READ, permissions.RESOURCE_WEBSITE
        )
        dag_tester_role = self.appbuilder.sm.find_role('dag_acl_tester')
        edit_perm_on_dag = self.appbuilder.sm.find_permission_view_menu(
            permissions.ACTION_CAN_EDIT, 'DAG:example_bash_operator'
        )
        self.appbuilder.sm.add_permission_role(dag_tester_role, edit_perm_on_dag)
        read_perm_on_dag = self.appbuilder.sm.find_permission_view_menu(
            permissions.ACTION_CAN_READ, 'DAG:example_bash_operator'
        )
        self.appbuilder.sm.add_permission_role(dag_tester_role, read_perm_on_dag)
        self.appbuilder.sm.add_permission_role(dag_tester_role, website_permission)
        all_dag_role = self.appbuilder.sm.find_role('all_dag_role')
        edit_perm_on_all_dag = self.appbuilder.sm.find_permission_view_menu(
            permissions.ACTION_CAN_EDIT, permissions.RESOURCE_DAG
        )
        self.appbuilder.sm.add_permission_role(all_dag_role, edit_perm_on_all_dag)
        read_perm_on_all_dag = self.appbuilder.sm.find_permission_view_menu(
            permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG
        )
        self.appbuilder.sm.add_permission_role(all_dag_role, read_perm_on_all_dag)
        self.appbuilder.sm.add_permission_role(all_dag_role, website_permission)