Fix concurrent error in cosmosdb example (#191)

* fix concurrent error in cosmosdb example

* change to random_string to reduce the collision rate

* fix 101-cosmos-db-free-tier account name too long issue

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

quickstart/101-cosmos-db-autoscale/main.tf

@@ -3,8 +3,21 @@ resource "azurerm_resource_group" "example" {
   location = var.location
 }
 
+resource "random_string" "db_account_name" {
+  count = var.cosmosdb_account_name == null ? 1 : 0
+
+  length  = 20
+  upper   = false
+  special = false
+  numeric = false
+}
+
+locals {
+  cosmosdb_account_name = try(random_string.db_account_name[0].result, var.cosmosdb_account_name)
+}
+
 resource "azurerm_cosmosdb_account" "example" {
-  name                      = var.cosmosdb_account_name
+  name                      = local.cosmosdb_account_name
   location                  = var.cosmosdb_account_location
   resource_group_name       = azurerm_resource_group.example.name
   offer_type                = "Standard"