Add Key Vault ID as a tag to virtual machines and enhance variable descriptions

Signed-off-by: Andre Licht <al@cloudeteer.de>

Author: lixhunter

README.md

@@ -816,7 +816,7 @@ Default: `null`
 Description: Whether to store generated secrets (admin password or SSH private key) in an Azure Key Vault.
 
 - If `true`, you must provide `key_vault_id`.
-- If `false`, `key_vault_id` must be null and no secrets will be stored.
+- If `false`, `key_vault_id` must be unset and no secrets will be stored.
 
 Type: `bool`