Adds an alarm action, represented as an SNS topic, to this alarm. 
        What do do when alarm is triggered.

        :type action_arn: str
        :param action_arn: SNS topics to which notification should be 
                           sent if the alarm goes to state ALARM.

def add_alarm_action(self, action_arn=None):
        """
        Adds an alarm action, represented as an SNS topic, to this alarm. 
        What do do when alarm is triggered.

        :type action_arn: str
        :param action_arn: SNS topics to which notification should be 
                           sent if the alarm goes to state ALARM.
        """
        if not action_arn:
            return # Raise exception instead?
        self.actions_enabled = 'true'
        self.alarm_actions.append(action_arn)