**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

tests/models/test_baseoperator.py
naive_datetime = DEFAULT_DATE.replace(tzinfo=None)
        op_no_dag = BaseOperator(
            task_id='test_task_naive_datetime', start_date=naive_datetime, end_date=naive_datetime
        )
        assert op_no_dag.start_date.tzinfo
        assert op_no_dag.end_date.tzinfo
    def test_setattr_performs_no_custom_action_at_execute_time(self):
        op = MockOperator(task_id="test_task")
        op_copy = op.prepare_for_execution()
        with mock.patch("airflow.models.baseoperator.BaseOperator.set_xcomargs_dependencies") as method_mock:
            op_copy.execute({})
        assert method_mock.call_count == 0
    def test_upstream_is_set_when_template_field_is_xcomarg(self):
        with DAG("xcomargs_test", default_args={"start_date": datetime.today()}):
            op1 = BaseOperator(task_id="op1")
            op2 = MockOperator(task_id="op2", arg1=op1.output)
        assert op1 in op2.upstream_list
        assert op2 in op1.downstream_list
    def test_set_xcomargs_dependencies_works_recursively(self):
        with DAG("xcomargs_test", default_args={"start_date": datetime.today()}):
            op1 = BaseOperator(task_id="op1")
            op2 = BaseOperator(task_id="op2")
            op3 = MockOperator(task_id="op3", arg1=[op1.output, op2.output])
            op4 = MockOperator(task_id="op4", arg1={"op1": op1.output, "op2": op2.output})
        assert op1 in op3.upstream_list
        assert op2 in op3.upstream_list
        assert op1 in op4.upstream_list
        assert op2 in op4.upstream_list
    def test_set_xcomargs_dependencies_works_when_set_after_init(self):