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Automatic merge from submit-queue (batch tested with PRs 60196, 59844, 60255, 60254, 60251). 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>. gce: allow extra addons to be sourced form a url This will allow istio configs to move out of the repo. Deleting https://github.com/kubernetes/kubernetes/tree/master/cluster/addons/istio will follow shortly. cc @ostromart @MrHohn ```release-note NONE ```