Use config sources for Kubelet, switch to podFullName

As discussed in #297 and also a side discussion about abstracting the Kubelet from being directly dependent on etcd, start refactoring config service to decouple sources of config from sync loop and switch from Manifest.ID -> Pod Full Name in the Kubelet.

Simplifies the Kubelet Run() step by requiring configuration to be defined up front, and an update channel passed into the Kubelet.
After looking at kubelet.go, I think I understand what you're trying to do here. Do you think you could push the goroutine starting into the NewPodConfig function? I just feel like it's going to be really hard for a person new to the system to track down where all the goroutines are started.