Return the mutable server application properties.

        :param key: the single property to return a value for
        :type key: Optional[str]
        :rtype: Union[Dict[str, str], List[Dict[str, str]]]

def application_properties(self, key=None):
        """Return the mutable server application properties.

        :param key: the single property to return a value for
        :type key: Optional[str]
        :rtype: Union[Dict[str, str], List[Dict[str, str]]]

        """
        params = {}
        if key is not None:
            params['key'] = key
        return self._get_json('application-properties', params=params)