Add logout command

"docker logout [SERVER]" will remove the registry server' credentials from
.dockercfg file. If a server is not specified, it will log user out of the
default docker registry server

Docker-DCO-1.1-Signed-off-by: Daniel, Dao Quang Minh <dqminh89@gmail.com> (github: dqminh)
Logout is a noun. Not a verb. Should be:

    Log out from the Docker Registry

We should also fix the previous login line at some point too.