Automatic merge from submit-queue (batch tested with PRs 65319, 64513, 65474, 65601, 65634). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.
Allow custom manifests in GCP master setup
Add a hook in GCE setup script to allow using custom manifests on master, so we can decouple some GKE changes from k8s. Note that this PR just adds a hook there is no change in default behavior.
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