Set the image build jobs to use the nodepool labels rather than
the nodeset definitions. This will allow us to switch nodeset
definitions back and forth between nodepool and niz easily and
quickly, without incurring the penalty of image builds.
Once we're further to the other side of the transition, we can
remove this and let the image builds use nodeset definitions.
Change-Id: I026df4d47a7a79d910d158fb0e8b8ceeb0045fd0
095cd1985e