Add RestartPolicy to Pod and PodTemplate

This is the first PR to add RestartPolicy to Pod. Next PRs will hook policy with kubelets. 
>However, about []error implementing Error() - Isn't returning multiple errors desirable in typical validation cases?

Sure, you'd basically be making an error tree. By default it should have a prettily printed message which lists all of the errors, but if you needed to, you could still get at the individual error objects via type assertions.