**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
"""Get Config option values from Secret Backend"""
    try:
        secrets_client = get_custom_secret_backend()
        if not secrets_client:
            return None
        return secrets_client.get_config(config_key)
    except Exception as e:  
        raise AirflowConfigException(
            'Cannot retrieve config from alternative secrets backend. '
            'Make sure it is configured properly and that the Backend '
            'is accessible.\n'
            f'{e}'
        )
def _default_config_file_path(file_name: str):
    templates_dir = os.path.join(os.path.dirname(__file__), 'config_templates')
    return os.path.join(templates_dir, file_name)
def default_config_yaml() -> List[dict]:
    """
    Read Airflow configs from YAML file
    :return: Python dictionary containing configs & their info
    """
    with open(_default_config_file_path('config.yml')) as config_file:
        return yaml.safe_load(config_file)
class AirflowConfigParser(ConfigParser):