The '*-simplified-installer' images require the installation device
to be specified as a BP customization to serialize their Manifest.
The 'describe' command was not setting those, which resulted in errors
when describing such image types.
Set a dummy installation device BP customization for image types with
their name ending with '-simplified-installer', to enable describing
of such images.
Signed-off-by: Tomáš Hozza <thozza@redhat.com>