Implement AwsBalancedZones constraint

[alanbover]

The default behavior of Amazon autoscaling group is to keep the instances balanced among the availability zones in a certain region (http://docs.aws.amazon.com/autoscaling/latest/userguide/as-instance-termination.html#default-termination-policy).

A constraint should be implemented to ensure the same behavior as the default AWS behavior for autoscaling groups