**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
"""Test render_template given various input types."""
        task = BaseOperator(task_id="op1")
        result = task.render_template(content, context)
        assert result == expected_output
    @pytest.mark.parametrize(
        ("content", "context", "expected_output"),
        [
            ("{{ foo }}", {"foo": "bar"}, "bar"),
            ("{{ foo }}", {"foo": ["bar1", "bar2"]}, ["bar1", "bar2"]),
            (["{{ foo }}", "{{ foo | length}}"], {"foo": ["bar1", "bar2"]}, [['bar1', 'bar2'], 2]),
            (("{{ foo }}_1", "{{ foo }}_2"), {"foo": "bar"}, ("bar_1", "bar_2")),
            ("{{ ds }}", {"ds": date(2018, 12, 6)}, date(2018, 12, 6)),
            (datetime(2018, 12, 6, 10, 55), {"foo": "bar"}, datetime(2018, 12, 6, 10, 55)),
            ("{{ ds }}", {"ds": datetime(2018, 12, 6, 10, 55)}, datetime(2018, 12, 6, 10, 55)),
            (MockNamedTuple("{{ foo }}_1", "{{ foo }}_2"), {"foo": "bar"}, MockNamedTuple("bar_1", "bar_2")),
            (
                ("{{ foo }}", "{{ foo.isoformat() }}"),
                {"foo": datetime(2018, 12, 6, 10, 55)},
                (datetime(2018, 12, 6, 10, 55), '2018-12-06T10:55:00'),
            ),
            (None, {}, None),
            ([], {}, []),
            ({}, {}, {}),
        ],
    )
    def test_render_template_with_native_envs(self, content, context, expected_output):