Tests and fixes for Elasticache VPC subnet groups

Based off @jtopper's work (PR #2403). Added some finishing touches to the models and removed status, because it's not actually a part of Elasticache subnet groups for some bizarre reason (see  http://docs.aws.amazon.com/AmazonElastiCache/latest/APIReference/API_CacheSubnetGroup.html vs http://docs.aws.amazon.com/AmazonRDS/latest/APIReference/API_DBSubnetGroup.html)