All of the current callers follow the pattern:
dockerPreRun(opts)
err := dockerCli.Initialize(opts) ...
So there is no semantic change into merging the content of `dockerPreRun` into the head of `Initialize`.
I'm about to add a new caller outside of the `cmd/docker` package and this
seems preferable exporting `DockerPreRun`.
Signed-off-by: Ian Campbell <ijc@docker.com>