Automatic merge from submit-queue
rkt: Fix panic in setting ReadOnlyRootFS
What the title says. I wish this method were broken out in a reasonably unit testable way. fixing this panic is more important for the second though, testing will come in a later commit.
I observed the panic in a `./hack/local-up-cluster.sh` run with rkt as the container runtime.
This is also the panic that's failing our jenkins against master ([recent run](https://console.cloud.google.com/m/cloudstorage/b/rktnetes-jenkins/o/logs/kubernetes-e2e-gce/1946/artifacts/jenkins-e2e-minion-group-qjh3/kubelet.log for the log output of a recent run))
cc @tmrts @yifan-gu