Passing pod UUID to Kubelet.

We need UUID to indicate the instance of a Pod to make restart work. 
This is to maintain the backward compatibility. There are more than one place to receive a pod request to Kubelet. With this PR, we make sure etcd's input has valid UUID, but we didn't make sure the pod from file, such as cadvisor. Also how about those running pods scheduled before this PR? I can break this, but thought it is nicer not do so by an arbitrary PR.