Adds an ok action, represented as an SNS topic, to this alarm. What
        to do when the ok state is reached.

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

def add_ok_action(self, action_arn=None):
        """
        Adds an ok action, represented as an SNS topic, to this alarm. What
        to do when the ok state is reached.

        :type action_arn: str
        :param action_arn: SNS topics to which notification should be 
                           sent if the alarm goes to state INSUFFICIENT_DATA.
        """
        if not action_arn:
            return
        self.actions_enabled = 'true'
        self.ok_actions.append(action_arn)