Add RestartPolicy to Pod and PodTemplate

This is the first PR to add RestartPolicy to Pod. Next PRs will hook policy with kubelets. 
Ahh, I agreed with moving this back to PodState. Like I mentioned above, I started from there, but moved it out due to a mistake. Since both DesiredState and CurrentState are PodState, somehow I thought CurrentState should include this field. But it should since the user might update the policy. 

I just did a rebase, not update the PR yet. Will ping you once I am done with it. But please don't wait for me. I am a late person. :-)