Config allows for various ways to authenticate Azure clients. When `client_id` and `subscription_id` are specified, Packer will use the specified Azure Active Directoty (AAD) Service Principal (SP). If only `subscription_id` is specified, Packer will try to interactively log on the current user (tokens will be cached). If none of these options are specified, Packer will attempt to use the Managed Identity and subscription of the VM that Packer is running on. This will only work if Packer is running on an Azure VM.