Automatic merge from submit-queue
dnsprovider: Add upsert
Although Google Cloud DNS requires strict add & remove calls, most
dnsproviders actually support upsert, and an add & remove is much more
expensive (primarily because of the need to fetch the pre-image).
Add support for 'upsert' operations, which don't require the pre-image,
and simply overwrite the existing record. This is much cheaper on
Amazon Route53, for example.
```release-note
NONE
```
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