**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
assert conf.get("core", "percent") == "with%inside"
        assert conf.get("core", "PERCENT") == "with%inside"
        assert conf.get("CORE", "PERCENT") == "with%inside"
    def test_config_as_dict(self):
        """Test that getting config as dict works even if
        environment has non-legal env vars"""
        with unittest.mock.patch.dict('os.environ'):
            os.environ['AIRFLOW__VAR__broken'] = "not_ok"
            asdict = conf.as_dict(raw=True, display_sensitive=True)
        assert asdict.get('VAR') is None
        assert asdict['testsection']['testkey'] == 'testvalue'
    def test_env_var_config(self):
        opt = conf.get('testsection', 'testkey')
        assert opt == 'testvalue'
        opt = conf.get('testsection', 'testpercent')
        assert opt == 'with%percent'
        assert conf.has_option('testsection', 'testkey')
        with unittest.mock.patch.dict(
            'os.environ', AIRFLOW__KUBERNETES_ENVIRONMENT_VARIABLES__AIRFLOW__TESTSECTION__TESTKEY='nested'
        ):
            opt = conf.get('kubernetes_environment_variables', 'AIRFLOW__TESTSECTION__TESTKEY')
            assert opt == 'nested'
    @mock.patch.dict(
        'os.environ', AIRFLOW__KUBERNETES_ENVIRONMENT_VARIABLES__AIRFLOW__TESTSECTION__TESTKEY='nested'
    )
    @conf_vars({("core", "percent"): "with%%inside"})
    def test_conf_as_dict(self):