Automatic merge from submit-queue

rkt: Map kubelet's `--stage1-image` flag to rkt's `--stage1-name` flag.

This enables rkt to use cached stage1 image instead of unpacking the stage1 image every time for every pod.

After this change, users need to preload the stage1 images in order to enable rkt to find the stage1 image with the name specified by this flag.

Also, the cloud config is modified to pre-load the stage1 images.

cc @kubernetes/sig-rktnetes @kubernetes/sig-node