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/variables.tf

@@ -12,7 +12,7 @@ variable "location" {
 
 variable "cosmosdb_account_name" {
   type        = string
-  default     = "default-cosmosdb-user"
+  default     = null
   description = "Cosmos db account name"
 }