**Description**

Enabled list of cors headers origin allowed

**Use case / motivation**

I'm currently developping Django application working with apache airflow for some external work through api. From Django, user run dag and a js request wait for dag end. By this way, I enabled cors headers in airflow.cfg with correct url. But now, I've  environment (dev & prod). It could be useful to be able to set more than one url in airflow.cfg cors header origin allowed

Thank's in advance :) 

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):