Add RestartPolicy to Pod and PodTemplate

This is the first PR to add RestartPolicy to Pod. Next PRs will hook policy with kubelets. 
@smarterclayton Good point! Will change it to a struct so that we could extend it easily. From current set of policies, you can see I didn't plan to have a policy which allows attempt restart max times. Also I plan to have a constant value as the minimum interval between restarts, but you are right about the user might want to override the default value here. 

@lavalamp I am ok with the terms you proposed above.

@brendanburns and @lavalamp I started from having RestartPolicy in PodState, but moved it out later.