@classmethod
def _build_ssm_key(cls, key):
    """Builds an SSM key.

        The format is "service_name/service_stage/key".

        Args:
            key: The key to build a fully qualified key for.

        Returns:
            The fully qualified key string.
        """
    service_name = cls.get('SERVICE_NAME', store=cls.Stores.OS).value
    service_stage = cls.get('SERVICE_STAGE', store=cls.Stores.OS).value
    if not service_name:
        raise ValueError('Environment variable not set: SERVICE_NAME')
    if not service_stage:
        raise ValueError('Environment variable not set: SERVICE_STAGE')
    return '/{0}/{1}/{2}'.format(service_name, service_stage, key)
