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
+  }
+}