Finally fixes subnet list output

This finally fixes the output coming from the vpc module so that it
returns a full list of subnets. Now they can be referenced just like the
redis module is using in this commit.

terraform/providers/dev/redis.tf

@@ -4,7 +4,7 @@ module "redis" {
   environment = var.environment
   region      = var.region
   name        = var.name
-  subnet_id   = module.vpc.subnets
+  subnet_id   = module.vpc.subnet_list["redis"].id
   sku_name    = "Premium"
   family      = "P"
 }