This is the minimal amount of work needed to be able to create a list of a subset of subnet IDs in a VPC, allowing people to loop through them easily when creating EC2 instances or provide a list straight to an ELB.
* add check destroy to subnet data source tests
* provider/aws: Fix TestAccAWSElasticacheCluster_snapshotsWithUpdates by waiting for snapshots
* provider/aws: Fix TestAccAWSRDSCluster_takeFinalSnapshot
* provider/aws: Fix TestAccAWSKinesisFirehoseDeliveryStream_ElasticsearchConfigUpdates by specifying instance type