The exec.Command code and os.Getenv implementation were both needlessly
verbose. This replaces them with simpler variations.
Signed-off-by: Euan Kemp <euank@euank.com>
It turns out the cred helpers protocol is to return "" and not an error
when a credential isn't present, so let's do that.
Signed-off-by: Tycho Andersen <tycho@docker.com>