Automatic merge from submit-queue. 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: Pass certificate URLs instead of the certificate structs
**What this PR does / why we need it**:
Simplify callers by passing in only what's necessary - a slice of URLs.
**Special notes for your reviewer**:
/assign @MrHohn
/cc @MrHohn
**Release note**:
```release-note
NONE
```
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