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
userContext holds a map with several contexts.  Should its name be plural or something.  Same question for UserRequestContext.