Allow configuring the rkt binary in a kubelet with a flag.

This is necessary because coreos comes with rkt installed, and if we want to use a different version
we need some way to avoid the default one.

hack/verify-flags/known-flags.txt

@@ -198,6 +198,7 @@ required-contexts
 resource-container
 resource-quota-sync-period
 resource-version
+rkt-path
 root-ca-file
 root-dir
 run-proxy