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.
I feel like this is a bit late in the game to do the validation. I think we should check at the control surfaces, not right before we're about to launch the containers. So for etcd, that'd be registry/pod_registry.go, and for the other methods, that'd be before the manifests are written to the update channel.