The original fix (#33147) sourced the correct `node-helper.sh` but set
`node_os_distribution` instead of `NODE_OS_DISTRIBUTION`. The
`set-node-image` function is imported indirectly via `source
"${KUBE_ROOT}/cluster/kube-util.sh"`, which in turn (in the GCE case)
sources `cluster/gce/util.sh`. Since the `set-node-image` function
relies on the `NODE_OS_DISTRIBUTION` variable, the original fix
did not have the entire intended effect.

I have confirmed that cherry-picking #33147 into the `release-1.4`
branch and layering this commit on top of it make for a successful
upgrade from a GCI based K8s 1.3 cluster to a GCI based K8s 1.4 cluster.