Change all references to the container ID in pkg/kubelet/... to the
strong type defined in pkg/kubelet/container: ContainerID
The motivation for this change is to make the format of the ID
unambiguous, specifically whether or not it includes the runtime
prefix (e.g. "docker://").
This change is part of the efforts to make DockerManager implement the Runtime
interface.
The change also modifies the interface slightly to work with existing
code, and aggregates the type converting functions to convert.go.
