**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

airflow/configuration.py
return {
            name: getattr(self, name)
            for name in [
                '_sections',
                'is_validated',
                'airflow_defaults',
            ]
        }
    def __setstate__(self, state):
        self.__init__()
        config = state.pop('_sections')
        self.read_dict(config)
        self.__dict__.update(state)
def get_airflow_home():
    """Get path to Airflow Home"""
    return expand_env_var(os.environ.get('AIRFLOW_HOME', '~/airflow'))
def get_airflow_config(airflow_home):