[diskimage-builder] bootable ISO image for baremetal

Hi! I know it is not one of the most wonderful options for DIB, but in some circumstances I used ISO image in a USB stick for some old and unmaintained hardware. I used to fix by hand every time the "iso" element to complete the task, due to an old reference to a deprecated variable (DIB_IMAGE_ROOT_FS_UUID, now DIB_BOOT_LABEL). I proposed a very small fix and I hope it could be merged in the future: https://review.opendev.org/c/openstack/diskimage-builder/+/905556 Thank you for all your great work. Cheers, Ettore