Don't expose KUBERNETES_VERSION to JJB

It was bad idea to expose KUBERNETES_VERSION var to integration
test JJB files because we have many of them, and we are now required
to add this var in almost every single integration test builder.
It is easier to control it from project-infra rather.

Change-Id: Ifd0c7ef9b109860ce91f1bfa3674c2f61ea85d89