Automatic merge from submit-queue

add rancher credential provider

This adds rancher as a credential provider in kubernetes.

@erictune This might be a good opportunity to discuss adding a provision for people to have their own credential providers that is similar to the new cloud provider changes (https://github.com/kubernetes/community/pull/128). WDYT?

```
release-note
Added Rancher Credential Provider to use Rancher Registry credentials when running in a Rancher cluster
```