**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
test_cmdenv_config = '''[testcmdenv]
itsacommand = NOT OK
notacommand = OK
'''
        test_cmdenv_conf = AirflowConfigParser()
        test_cmdenv_conf.read_string(test_cmdenv_config)
        test_cmdenv_conf.sensitive_config_values.add(('testcmdenv', 'itsacommand'))
        with unittest.mock.patch.dict('os.environ'):
            
            
            
            assert test_cmdenv_conf.get('testcmdenv', 'itsacommand') == 'OK'
            
            
            
            assert test_cmdenv_conf.get('testcmdenv', 'notacommand') == 'OK'
    def test_parameterized_config_gen(self):