Add sample for iothub with private link

quickstart/201-private-link-iothub-builtin-endpoint/variables.tf

@@ -0,0 +1,17 @@
+variable "location" {
+    type = string
+    default = "westeurope"
+    description = "Location of the resource group"
+}
+
+variable "vnet_address_space" {
+    type = string
+    default = "10.0.0.0/16"
+    description = "Address range of the virtual network"
+}
+
+variable "iothub_subnet_address_space" {
+    type = string
+    default = "10.0.3.0/24"
+    description = "Address range of the subnet containing the iothub"
+}