fix test code output issue.

change `cluster_identity` type from list to object so the user won't need use index anymore.

Author: lonegunmanb

outputs.tf

@@ -55,7 +55,7 @@ output "http_application_routing_zone_name" {
 }
 
 output "cluster_identity" {
-  value = azurerm_kubernetes_cluster.main.identity
+  value = try(azurerm_kubernetes_cluster.main.identity[0], null)
 }
 
 output "kubelet_identity" {

test/fixture/outputs.tf

@@ -7,7 +7,7 @@ output "test_aks_without_monitor_id" {
 }
 
 output "test_aks_without_monitor_identity" {
-  value = module.aks_without_monitor.system_assigned_identity
+  value = module.aks_without_monitor.cluster_identity
 }
 
 output "test_admin_client_key" {