Add simple Bearer authenticator filter for Kube

Now with 100% more implementation. Step 1 of implementing #1430.  Sourced from https://github.com/smarterclayton/origin/commit/b3ccda08e51a4b129348244e7b6c03ccc54750f3

@erictune
Yeah.

----- Original Message -----
> > +Licensed under the Apache License, Version 2.0 (the "License");
> > +you may not use this file except in compliance with the License.
> > +You may obtain a copy of the License at
> > +
> > +    http://www.apache.org/licenses/LICENSE-2.0
> > +
> > +Unless required by applicable law or agreed to in writing, software
> > +distributed under the License is distributed on an "AS IS" BASIS,
> > +WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
> > +See the License for the specific language governing permissions and
> > +limitations under the License.
> > +*/
> > +
> > +package api
> > +
> > +type DefaultUserInfo struct {
> 
> It was a little confusing that its name is api.DefaultUserInfo, but it is not
> a Kubernetes API object.  Put in a different package?
> 
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> https://github.com/GoogleCloudPlatform/kubernetes/pull/1529/files#r18408398