Automatic merge from submit-queue (batch tested with PRs 41937, 41151, 42092, 40269, 42135)
[Federation] Make --dns-provider mandatory for kubefed
Targets #40757
I thought about adding a test for this but I decided it wasn't worth it. There's too much setup involved in being able to run ``Complete`` for such a simple change.
**Release note**:
```release-note
The --dns-provider argument of 'kubefed init' is now mandatory and does not default to `google-clouddns`. To initialize a Federation control plane with Google Cloud DNS, use the following invocation: 'kubefed init --dns-provider=google-clouddns'
```
cc: @kubernetes/sig-federation-pr-reviews @madhusudancs
a7d9418c08