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
```