uncomment backend (#1327)

cluster/environments/azure-single-keyvault/main.tf

@@ -1,6 +1,7 @@
-#terraform {
-#  backend "azurerm" {}
-#}
+terraform {
+  backend "azurerm" {}
+}
+
 module "provider" {
   source = "github.com/microsoft/bedrock?ref=master//cluster/azure/provider"
 }