Automatic merge from submit-queue (batch tested with PRs 61644, 61624, 61743, 61019, 61287). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

provide easy methods for direct kubeconfig loading from bytes

Adds a `RESTConfigFromKubeConfig([]byte)` method for taking a kubeconfig and getting back the rest.Config.  There are ways to do this now, but it takes a fair amount of wiring that is a pain.

As kube starts dropping `--master` flags from its commands, it will be able to use this.  For current consumers, this will be a big simplification.

```release-note
NONE
```