WIP: First cut at input validation for Kubelet

* Add some util functions for basic validation
* Spec api fields more tightly
* Break a dep from util -> api, so I can do the reverse
* Add functions to validate structures
* Call those functions when running manifests

This does not validate as it unmarshals YAML or JSON because I want to be able
to validate structures that come from other inputs without forcing them to be
YAML.

This does not handle all api structures - I am not familiar wnough with the
rest to spec them this tightly - I hope someone else can do that.

There are a number of open questions, marked "FIXME".  Please offer feedback
or opinions.
We should check both places. My concern with it as written is that kubelet silently drops the pod, at least from apiserver's perspective. We need a mechanism for kubelet to formally reject a pod, because right now the apiserver is just going to think it's reallllllly slow at starting the pod.