**Description**

It would be nice if the weight_rule default value could be managed by a global config

suggested config:
```
# Weighting method used for the effective total priority weight of the task. 
# Options are: { downstream | upstream | absolute } default is
default_weight_rule = downstream
```
**Use case / motivation**

In some pipeline, you really need to have absolute weight, and then you have to add a line in each task definition which is annoying

tests/core/test_configuration.py
'created_time': '2020-03-28T02:10:54.301784Z',
                    'deletion_time': '',
                    'destroyed': False,
                    'version': 1,
                },
            },
            'wrap_info': None,
            'warnings': None,
            'auth': None,
        }
        test_config = '''[test]
sql_alchemy_conn_secret = sql_alchemy_conn
'''
        test_config_default = '''[test]
sql_alchemy_conn = airflow
'''
        test_conf = AirflowConfigParser(default_config=parameterized_config(test_config_default))
        test_conf.read_string(test_config)
        test_conf.sensitive_config_values = test_conf.sensitive_config_values | {
            ('test', 'sql_alchemy_conn'),
        }
        assert 'sqlite:////Users/airflow/airflow/airflow.db' == test_conf.get('test', 'sql_alchemy_conn')
    @mock.patch("airflow.providers.hashicorp._internal_client.vault_client.hvac")
    @conf_vars(
        {
            ("secrets", "backend"): "airflow.providers.hashicorp.secrets.vault.VaultBackend",
            ("secrets", "backend_kwargs"): '{"url": "http://127.0.0.1:8200", "token": "token"}',
        }
    )
    def test_config_raise_exception_from_secret_backend_connection_error(self, mock_hvac):