**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
key = env_var.replace(section_prefix, '')
                if key.endswith("_CMD"):
                    key = key[:-4]
                key = key.lower()
                _section[key] = self._get_env_var_option(section, key)
        for key, val in _section.items():
            try:
                val = int(val)
            except ValueError:
                try:
                    val = float(val)
                except ValueError:
                    if val.lower() in ('t', 'true'):
                        val = True
                    elif val.lower() in ('f', 'false'):
                        val = False
            _section[key] = val
        return _section
    def write(self, fp, space_around_delimiters=True):
        
        
        if space_around_delimiters:
            delimiter = f" {self._delimiters[0]} "
        else:
            delimiter = self._delimiters[0]
        if self._defaults:
            self._write_section(fp, self.default_section, self._defaults.items(), delimiter)
        for section in self._sections:
            self._write_section(fp, section, self.getsection(section).items(), delimiter)
    def as_dict(