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/modules/vpc/outputs.tf

@@ -1,3 +1,9 @@
 output "subnets" {
-  value = azurerm_subnet.subnet["private"].id #FIXME - output should be a map
+  value = azurerm_subnet.subnet["private"].id #FIXED: this is now legacy, use subnet_list
 }
+
+output "subnet_list" {
+  value = {
+    for k, id in azurerm_subnet.subnet : k => id
+  }
+}

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"
 }