Add RestartPolicy to Pod and PodTemplate

This is the first PR to add RestartPolicy to Pod. Next PRs will hook policy with kubelets. 
How are arguments to a restart policy passed (specifically max retries or minimum duration between restarts)?  Is might be better as a struct with a single field "type" if we know we will add those additional constants.  The alternative is RestartPolicy and optional *RestartPolicyArguments - did you have a direction in mind?