**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/models/baseoperator.py
be set by using the set_upstream and/or set_downstream methods.
    :param task_id: a unique, meaningful id for the task
    :type task_id: str
    :param owner: the owner of the task. Using a meaningful description
        (e.g. user/person/team/role name) to clarify ownership is recommended.
    :type owner: str
    :param email: the 'to' email address(es) used in email alerts. This can be a
        single email or multiple ones. Multiple addresses can be specified as a
        comma or semi-colon separated string or by passing a list of strings.
    :type email: str or list[str]
    :param email_on_retry: Indicates whether email alerts should be sent when a
        task is retried
    :type email_on_retry: bool
    :param email_on_failure: Indicates whether email alerts should be sent when
        a task failed
    :type email_on_failure: bool
    :param retries: the number of retries that should be performed before
        failing the task
    :type retries: int
    :param retry_delay: delay between retries
    :type retry_delay: datetime.timedelta
    :param retry_exponential_backoff: allow progressive longer waits between
        retries by using exponential backoff algorithm on retry delay (delay
        will be converted into seconds)
    :type retry_exponential_backoff: bool
    :param max_retry_delay: maximum delay interval between retries
    :type max_retry_delay: datetime.timedelta
    :param start_date: The ``start_date`` for the task, determines
        the ``execution_date`` for the first task instance. The best practice
        is to have the start_date rounded