Automatic merge from submit-queue

rkt: Fix incomplete selinux context string when the option is partial.

Fix "EmptyDir" e2e tests failures caused by #https://github.com/kubernetes/kubernetes/pull/24901

As mentioned in https://github.com/kubernetes/kubernetes/pull/24901#discussion_r61372312
We should apply the selinux context of the rkt data directory (/var/lib/rkt) when users do not specify all the selinux options.

Due to my fault, the change was missed during rebase, thus caused the regression.

After applying this PR, the e2e tests passed.
```
$ go run hack/e2e.go -v -test --test_args="--ginkgo.dryRun=false --ginkgo.focus=EmptyDir"
...
Ran 19 of 313 Specs in 199.319 seconds
SUCCESS! -- 19 Passed | 0 Failed | 0 Pending | 294 Skipped PASS
```

BTW, the test is removed because the `--no-overlay=true` flag will only be there on non-coreos distro.

cc @euank @kubernetes/sig-node