201-confidential-vm patch (#146)

* fix 201-confidential-vm example

---------

Co-authored-by: zjhe <hezijie@microsoft.com>

quickstart/201-confidential-vm/variables.tf

@@ -1,14 +1,11 @@
 variable "location" {
   type        = string
+  default     = "eastus"
   description = "Location where resources will be created"
 }
 
-variable "name_prefix" {
-  type        = string
-  description = "Prefix of the resource name"
-}
-
-variable "vm_public_key" {
-  type        = string
-  description = "Public key of the Virtual Machine"
-}
+variable "msi_id" {
+  type = string
+  default = null
+  description = "If you're executing the test with user assigned identity, please pass the identity principal id to this variable."
+}