[FYI] Category for livecd related jobs

Hey, recently we've seen some build problems related to all loop devices beeing in use. This originated (very likely) from multiple concurrent jobs running on the same node and using livecd-tools (e.g. livecd-creator) which exhausted all available loop devices. I discussed this with Mike and he suggested to introduce a category (iso_build) which restricts multiple jobs of this same category to be build at once on the same builder. To take advantage of this I added the category (iso_build) to all Node related jobs (ovirt-node-iso, ovirt-node-iso-devel, ovirt-node-iso-fedora-next, and ovirt-node-iso-edit-node) requiring livecd-tools. Mike added the category to the ovirt-live job, which is also using livecd-tools. This should solve - at least - the loop device related problems. Greetings fabian