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.
Magic number corresponding to approximate number of realistic sources.  Can make it less magic (1).