Clarify the error message returned when trying to use the docker runtime
on a Kubelet that was compiled without Docker.
We removed the "w/" and "w/o", which can be confusing abbreviations, and
also add slightly more detail on the actual error.
Extract a `runDockershim` function into a file outside of `kubelet.go`.
We can use build tags to compile two separate functions... one which
actually runs dockershim and one that is a no-op.