**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.has_option("core", "FERNET_KEY")
        assert not conf.has_option("core", "FERNET_KEY_CMD")
        fernet_key = conf.get('core', 'FERNET_KEY')
        with conf_vars({('core', 'FERNET_KEY_CMD'): 'printf HELLO'}):
            fallback_fernet_key = conf.get("core", "FERNET_KEY")
        assert fernet_key == fallback_fernet_key
    def test_config_throw_error_when_original_and_fallback_is_absent(self):
        assert conf.has_option("core", "FERNET_KEY")
        assert not conf.has_option("core", "FERNET_KEY_CMD")
        with conf_vars({('core', 'fernet_key'): None}):
            with pytest.raises(AirflowConfigException) as ctx:
                conf.get("core", "FERNET_KEY")
        exception = str(ctx.value)
        message = "section/key [core/fernet_key] not found in config"
        assert message == exception
    def test_config_override_original_when_non_empty_envvar_is_provided(self):