Generate iso files according to inventory `boot_iso`

[dbutenhof]

The generate-discovery-iso role will always create discovery.iso rather than using the inventory boot_iso name, but the boot_iso (sub) roles boot using the boot_iso hostvar.

The standard inventory creators always set boot_iso to "discovery.iso", but if a BYOL inventory sets boot_iso to something else, the boot-iso role will try to use it and won't find it.

We should be consistent about this, and use boot_iso in both places.

Originally posted by @dbutenhof in https://github.com/redhat-performance/jetlag/pull/489#discussion_r1594161374