This pushes artifacts in a similar manner to the official release,
except that instead of release/vFOO, it goes to ci/$(git describe),
e.g.: gs://kubernetes-release/ci/v0.7.0-315-gcae5722

It also pushes a text file to gs://kubernetes-release/ci/latest.txt,
so anyone can do, for instance:

gsutil ls gs://kubernetes-release/ci/$(gsutil cat gs://kubernetes-release/ci/latest.txt)

(In a parallel change, I'm going to flip the jenkins scripts over to
use git describe, since it's shorter and a little more descriptive)