If the image with :latest tag specified in Spec, kubelet should try

to pull the latest one even the policy is PullIfNotPresent.

My initial thought is that we should inspect ContainerSpec and set proper default based on image tag. But it looks like not very easy once we move all default setting to conversion. For now, I just put it to Kubelet, but I am open to other suggestion. 

@lavalamp @brendandburns