Use `zuul.image_build_name` as default for the
`build_diskimage_image_name` variable, so that is enough for jobs to
define `job.image-build-name`.
If `zuul.image_build_name` is not set we require
`build_diskimage_image_name`to be defined.
Change-Id: I837b5d1571dce38801d6a0433e960734f573fedc