Automatic merge from submit-queue (batch tested with PRs 46512, 50146) Default NODE_OS to gci on GCE installs Looking at https://github.com/kubernetes/test-infra/tree/master/jobs, any job with "cvm" in the name explicitly sets this variable, so I hope it is only the default. **Release note**: ```release-note Change GCE installs (kube-up.sh) to use GCI/COS for node OS, by default. ``` @mtaufen @dchen1107 @ixdy