Automatic merge from submit-queue

Define interfaces for kubelet pod admission and eviction

There is too much code and logic in `kubelet.go` that makes it hard to test functions in discrete pieces.

I propose an interface that an internal module can implement that will let it make an admission decision for a pod.  If folks are ok with the pattern, I want to move the a) predicate checking, b) out of disk, c) eviction preventing best-effort pods being admitted into their own dedicated handlers that would be easier for us to mock test.  We can then just write tests to ensure that the `Kubelet` calls a call-out, and we can write easier unit tests to ensure that dedicated handlers do the right thing.

The second interface I propose was a `PodEvictor` that is invoked in the main kubelet sync loop to know if pods should be pro-actively evicted from the machine.  The current active deadline check should move into a simple evictor implementation, and I want to plug the out of resource killer code path as an implementation of the same interface.

 @vishh @timothysc - if you guys can ack on this, I will add some unit testing to ensure we do the call-outs.

/cc @kubernetes/sig-node @kubernetes/rh-cluster-infra