**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
"""Custom Airflow Configparser supporting defaults and deprecated options"""
    
    
    
    
    
    sensitive_config_values = {
        ('core', 'sql_alchemy_conn'),
        ('core', 'fernet_key'),
        ('celery', 'broker_url'),
        ('celery', 'flower_basic_auth'),
        ('celery', 'result_backend'),
        ('atlas', 'password'),
        ('smtp', 'smtp_password'),
        ('webserver', 'secret_key'),
    }
    
    
    
    deprecated_options = {
        ('celery', 'worker_precheck'): ('core', 'worker_precheck', '2.0.0'),
        ('logging', 'base_log_folder'): ('core', 'base_log_folder', '2.0.0'),
        ('logging', 'remote_logging'): ('core', 'remote_logging', '2.0.0'),
        ('logging', 'remote_log_conn_id'): ('core', 'remote_log_conn_id', '2.0.0'),
        ('logging', 'remote_base_log_folder'): ('core', 'remote_base_log_folder', '2.0.0'),
        ('logging', 'encrypt_s3_logs'): ('core', 'encrypt_s3_logs', '2.0.0'),
        ('logging', 'logging_level'): ('core', 'logging_level', '2.0.0'),
        ('logging', 'fab_logging_level'): ('core', 'fab_logging_level', '2.0.0'),
        ('logging', 'logging_config_class'): ('core', 'logging_config_class', '2.0.0'),
        ('logging', 'colored_console_log'): ('core', 'colored_console_log', '2.0.0'),