Add dependencies between subnets and dat sources

Without an explicit dependency between the `created_subnets` data source
and the `subnetwork` resource, the data source may be fetched before
subnets are created, causing stale information to be provided. This
commit resolves the issue by using resource attributes within the data
source, creating the necessary dependency.

Author: adrienthebo

main.tf

@@ -52,8 +52,8 @@ resource "google_compute_subnetwork" "subnetwork" {
 data "google_compute_subnetwork" "created_subnets" {
   count = "${length(var.subnets)}"
 
-  name    = "${lookup(var.subnets[count.index], "subnet_name")}"
-  region  = "${lookup(var.subnets[count.index], "subnet_region")}"
+  name    = "${element(google_compute_subnetwork.subnetwork.*.name, count.index)}"
+  region  = "${element(google_compute_subnetwork.subnetwork.*.region, count.index)}"
   project = "${var.project_id}"
 }