**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
:param on_execute_callback: much like the ``on_failure_callback`` except
        that it is executed right before the task is executed.
    :type on_execute_callback: TaskStateChangeCallback
    :param on_retry_callback: much like the ``on_failure_callback`` except
        that it is executed when retries occur.
    :type on_retry_callback: TaskStateChangeCallback
    :param on_success_callback: much like the ``on_failure_callback`` except
        that it is executed when the task succeeds.
    :type on_success_callback: TaskStateChangeCallback
    :param pre_execute: a function to be called immediately before task
        execution, receiving a context dictionary; raising an exception will
        prevent the task from being executed.
        |experimental|
    :type pre_execute: TaskPreExecuteHook
    :param post_execute: a function to be called immediately after task
        execution, receiving a context dictionary and task result; raising an
        exception will prevent the task from succeeding.
        |experimental|
    :type post_execute: TaskPostExecuteHook
    :param trigger_rule: defines the rule by which dependencies are applied
        for the task to get triggered. Options are:
        ``{ all_success | all_failed | all_done | one_success |
        one_failed | none_failed | none_failed_min_one_success | none_skipped | always}``
        default is ``all_success``. Options can be set as string or
        using the constants defined in the static class
        ``airflow.utils.TriggerRule``
    :type trigger_rule: str
    :param resources: A map of resource parameter names (the argument names of the
        Resources constructor) to their values.
    :type resources: dict