**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
"""
    Load the Airflow config files.
    Called for you automatically as part of the Airflow boot process.
    """
    global FERNET_KEY, AIRFLOW_HOME
    default_config = _parameterized_config_from_template('default_airflow.cfg')
    conf = AirflowConfigParser(default_config=default_config)
    if conf.getboolean('core', 'unit_test_mode'):
        
        if not os.path.isfile(TEST_CONFIG_FILE):
            from cryptography.fernet import Fernet
            log.info('Creating new Airflow config file for unit tests in: %s', TEST_CONFIG_FILE)
            pathlib.Path(AIRFLOW_HOME).mkdir(parents=True, exist_ok=True)
            FERNET_KEY = Fernet.generate_key().decode()
            with open(TEST_CONFIG_FILE, 'w') as file:
                cfg = _parameterized_config_from_template('default_test.cfg')
                file.write(cfg)
        conf.load_test_config()
    else:
        
        if not os.path.isfile(AIRFLOW_CONFIG):
            from cryptography.fernet import Fernet
            log.info('Creating new Airflow config file in: %s', AIRFLOW_CONFIG)
            pathlib.Path(AIRFLOW_HOME).mkdir(parents=True, exist_ok=True)
            FERNET_KEY = Fernet.generate_key().decode()
            with open(AIRFLOW_CONFIG, 'w') as file:
                file.write(default_config)
        log.info("Reading the config from %s", AIRFLOW_CONFIG)
        conf.read(AIRFLOW_CONFIG)
        if conf.has_option('core', 'AIRFLOW_HOME'):