Automatic merge from submit-queue
Kubeadm discovery remove error passing
**What this PR does / why we need it**: In the app/discovery there is some confusion about the passing of error values created in the discovery/token, discovery/https/ and discovery/file pkgs. Since they always return `nil` , it was very confusing in discovery/flags.go why to propagate them up as if there was a chance for them to return a value other than `nil`. This change makes it much more clear what is being passed.
I noticed this as I was making a sweep through trying to add more unit tests and it was very confusing to read the code.
**Which issue this PR fixes** : fixes #https://github.com/kubernetes/kubeadm/issues/141
**Special notes for your reviewer**: /cc @luxas @pires
**Release note**:
```release-note
NONE
```
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