[aws|elb] support for cross zone load balancing

Support for cross zone load balancing, as described here: http://docs.aws.amazon.com/ElasticLoadBalancing/latest/DeveloperGuide/TerminologyandKeyConcepts.html#AZ-Region

I'm a little uncertain about the model side of things - should I be caching the value? It's a slightly odd value in that it feels like an attribute of the load balancer, but as far as I can tell it is only returned by describe_load_balancer_attributes, not by describe_load_balancers