**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
cfg_dict = conf.as_dict(include_env=False, display_sensitive=True)
        
        
        assert 'testsection' not in cfg_dict
    def test_command_precedence(self):
        test_config = '''[test]
key1 = hello
key2_cmd = printf cmd_result
key3 = airflow
key4_cmd = printf key4_result
'''
        test_config_default = '''[test]
key1 = awesome
key2 = airflow
[another]
key6 = value6
'''
        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', 'key2'),
            ('test', 'key4'),
        }
        assert 'hello' == test_conf.get('test', 'key1')
        assert 'cmd_result' == test_conf.get('test', 'key2')
        assert 'airflow' == test_conf.get('test', 'key3')