Automatic merge from submit-queue. 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>.

Runtimeclass podapi

Adds (but doesn't implement) the RuntimeClass Pod API proposed in https://github.com/kubernetes/community/blob/master/keps/sig-node/0014-runtime-class.md

Based on the result (lazy consensus) of the alpha fields discussion on sig-architecture: https://groups.google.com/forum/#!topic/kubernetes-sig-architecture/y9FulL9Uq6A

For https://github.com/kubernetes/features/issues/585

**Special notes for your reviewer**:

**Release note**:
```release-note
RuntimeClass is a new API resource for defining different classes of runtimes that may be used to run containers in the cluster. Pods can select a RunitmeClass to use via the RuntimeClassName field. This feature is in alpha, and the RuntimeClass feature gate must be enabled in order to use it.
```

/sig node
/kind api-change
/priority important-soon
/milestone v1.12