Automatic merge from submit-queue

Start moving genericapiserver to staging

This moves `pkg/auth/user` to `staging/k8s.io/genericapiserver/pkg/authentication/user`.  I'll open a separate pull into the upstream gengo to support using `import-boss` on vendored folders to support staging.

After we agree this is the correct approach and see everything build, I'll start moving other packages over which don't have k8s.io/kubernetes deps.

@kubernetes/sig-api-machinery-misc @lavalamp 
@sttts @caesarxuchao ptal