Update ansible inventory file when template changes (#1612)

This trigger ensures the inventory file is kept up-to-date. Otherwise, if the file exists and you've made changes to your terraform-managed infra without having deleted the file, it would never get updated. 

For example, consider the case where you've destroyed and re-applied the terraform resources, none of the IPs would get updated, so ansible would be trying to connect to the old ones.

contrib/terraform/aws/create-infrastructure.tf

@@ -162,7 +162,7 @@ resource "aws_instance" "k8s-worker" {
 */
 data "template_file" "inventory" {
     template = "${file("${path.module}/templates/inventory.tpl")}"
-
+  
     vars {
         public_ip_address_bastion = "${join("\n",formatlist("bastion ansible_ssh_host=%s" , aws_instance.bastion-server.*.public_ip))}"
         connection_strings_master = "${join("\n",formatlist("%s ansible_ssh_host=%s",aws_instance.k8s-master.*.tags.Name, aws_instance.k8s-master.*.private_ip))}"
@@ -176,6 +176,7 @@ data "template_file" "inventory" {
         kube_insecure_apiserver_address = "kube_apiserver_insecure_bind_address: ${var.kube_insecure_apiserver_address}"
 
     }
+
 }
 
 resource "null_resource" "inventories" {
@@ -183,4 +184,8 @@ resource "null_resource" "inventories" {
       command = "echo '${data.template_file.inventory.rendered}' > ../../../inventory/hosts"
   }
 
+  triggers {
+      template = "${data.template_file.inventory.rendered}"
+  }
+
 }