Add new Instances to an existing Load Balancer.

        :type load_balancer_name: string
        :param load_balancer_name: The name of the Load Balancer

        :type instances: List of strings
        :param instances: The instance ID's of the EC2 instances to add.

        :rtype: List of strings
        :return: An updated list of instances for this Load Balancer.

def register_instances(self, load_balancer_name, instances):
        """
        Add new Instances to an existing Load Balancer.

        :type load_balancer_name: string
        :param load_balancer_name: The name of the Load Balancer

        :type instances: List of strings
        :param instances: The instance ID's of the EC2 instances to add.

        :rtype: List of strings
        :return: An updated list of instances for this Load Balancer.

        """
        params = {'LoadBalancerName' : load_balancer_name}
        self.build_list_params(params, instances,
                               'Instances.member.%d.InstanceId')
        return self.get_list('RegisterInstancesWithLoadBalancer',
                             params, [('member', InstanceInfo)])