**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
conf.deprecated_options[('celery', "result_backend")] = 'celery', 'celery_result_backend', '2.0.0'
        conf.sensitive_config_values.add(('celery', 'celery_result_backend'))
        conf.remove_option('celery', 'result_backend')
        with conf_vars({('celery', 'celery_result_backend_cmd'): '/bin/echo 99'}):
            with pytest.warns(DeprecationWarning):
                tmp = None
                if 'AIRFLOW__CELERY__RESULT_BACKEND' in os.environ:
                    tmp = os.environ.pop('AIRFLOW__CELERY__RESULT_BACKEND')
                assert conf.getint('celery', 'result_backend') == 99
                if tmp:
                    os.environ['AIRFLOW__CELERY__RESULT_BACKEND'] = tmp
    def test_deprecated_values_from_conf(self):