multi_disk: Fix incorrect params

* max_disk must not have any usbs, otherwise there are not enough
  slots in PCI
* default image_boot = yes which generates in multi_disk test
  multiple disks with index = 1 and test failure. By forcing default
  value stg_image_boot = 'no' we can keep the default value (and index
  of image1) and let the other disk to have their own indexes.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>