**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/configuration.py
config, config_sources, display_source, raw, section, source_name
    ):
        sect = config_sources.setdefault(section, OrderedDict())
        for (k, val) in config.items(section=section, raw=raw):
            if display_source:
                val = (val, source_name)
            sect[k] = val
    def load_test_config(self):
        """
        Load the unit test configuration.
        Note: this is not reversible.
        """
        
        for section in self.sections():
            self.remove_section(section)
        
        path = _default_config_file_path('default_test.cfg')
        log.info("Reading default test configuration from %s", path)
        self.read_string(_parameterized_config_from_template('default_test.cfg'))
        
        log.info("Reading test configuration from %s", TEST_CONFIG_FILE)
        self.read(TEST_CONFIG_FILE)
    @staticmethod
    def _warn_deprecate(section, key, deprecated_section, deprecated_name):