Cleaning up container ID handling inside kubelet

This touches a lot of stuff, so I'd appreciate a careful review & comments!

In  another PR, I'd like to improve the /containerInfo and /containerStats interface. I maintained the previous behavior of passing a manifest/pod id (which really ends up being a substring match in the mangled docker name!). This seems to fall short since a manifest/pod can container multiple containers.
I'm still new to go, so grain of salt: What do you think about doing:

type DockerID string and then using that type everywhere?