**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
"""
        Override this method to cleanup subprocesses when a task instance
        gets killed. Any use of the threading, subprocess or multiprocessing
        module within an operator needs to be cleaned up or it will leave
        ghost processes behind.
        """
    def __deepcopy__(self, memo):
        """
        Hack sorting double chained task lists by task_id to avoid hitting
        max_depth on deepcopy operations.
        """
        sys.setrecursionlimit(5000)  
        cls = self.__class__
        result = cls.__new__(cls)
        memo[id(self)] = result
        shallow_copy = cls.shallow_copy_attrs + cls._base_operator_shallow_copy_attrs
        for k, v in self.__dict__.items():
            if k not in shallow_copy:
                setattr(result, k, copy.deepcopy(v, memo))
            else:
                setattr(result, k, copy.copy(v))
        return result
    def __getstate__(self):
        state = dict(self.__dict__)
        del state['_log']
        return state
    def __setstate__(self, state):