[FYI] Category for livecd related jobs

[Fabian Deutsch]

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