Add RestartPolicy to Pod and PodTemplate

This is the first PR to add RestartPolicy to Pod. Next PRs will hook policy with kubelets. 
I agreed with @smarterclayton on returning multiple errors is much more desirable in validation than a single error. 

On the issue of using append directly or wrapping it in Append. I don't have a preference, I just simply follow the local idiom here. I think the consistency in a file / project is very important for readability and understandability too. In validation.go, only one place use append directly, rest are using Append. I could go either way, but prefer to change it in a separate PR and make sure the consistency in the entire file.