Automatic merge from submit-queue
Use Container-optimzed OS images for nodes by default
Part of the deprecation of the debian-based ContainerVM images.
```release-note
kube-up and kubemark will default to using cos (GCI) images for nodes.
The previous default was container-vm (CVM, "debian"), which is deprecated.
If you need to explicitly use container-vm for some reason, you should set
KUBE_NODE_OS_DISTRIBUTION=debian
```
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