adapt the debian buster template to be used by terraform

• The current template is configured with a default lsi scsi adaptor and a scsi disk. Terraform is using a virtio disk and creates a new virtio disk one if not existing instead of extended the current scsi volume. Configuring the template wirh a virtio disk solves the problem.

• a qemu-server upgrade has changed the boot order option behavior which led to an wrong definition of the boot devices by packer. It forces to perform a manual action during the template creation and to explicitly declare the cloud_init_storage_pool option because the default value is computed from the now not existing boot device. It will be fixed on a future packer release1.

Related to T2747

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Migrated from D4422 (view on Phabricator)