**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_config = '''[test]
key1 = value1
'''
        test_conf = AirflowConfigParser()
        test_conf.read_string(test_config)
        assert test_conf.has_option('test', 'key1')
        assert not test_conf.has_option('test', 'key_not_exists')
        assert not test_conf.has_option('section_not_exists', 'key1')
    def test_remove_option(self):
        test_config = '''[test]
key1 = hello
key2 = airflow
'''
        test_config_default = '''[test]
key1 = awesome
key2 = airflow
'''
        test_conf = AirflowConfigParser(default_config=parameterized_config(test_config_default))
        test_conf.read_string(test_config)
        assert 'hello' == test_conf.get('test', 'key1')
        test_conf.remove_option('test', 'key1', remove_default=False)
        assert 'awesome' == test_conf.get('test', 'key1')
        test_conf.remove_option('test', 'key2')
        assert not test_conf.has_option('test', 'key2')
    def test_getsection(self):