Add RestartPolicy to Pod and PodTemplate

This is the first PR to add RestartPolicy to Pod. Next PRs will hook policy with kubelets. 
Agree about append.  However, about []error implementing Error() - Isn't returning multiple errors desirable in typical validation cases?  A typical function has many different errors coming from multiple sources, but usually only one fails at a time - validation is the opposite, find all errors in this structure.