**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
val = self.get(section, key, **kwargs)
        try:
            return int(val)
        except ValueError:
            raise AirflowConfigException(
                f'Failed to convert value to int. Please check "{key}" key in "{section}" section. '
                f'Current value: "{val}".'
            )
    def getfloat(self, section, key, **kwargs):
        val = self.get(section, key, **kwargs)
        try:
            return float(val)
        except ValueError:
            raise AirflowConfigException(
                f'Failed to convert value to float. Please check "{key}" key in "{section}" section. '
                f'Current value: "{val}".'
            )
    def getimport(self, section, key, **kwargs):