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)
I'm setting it to 0 because for end-user, the effect will be the same (i.e. serverAddress will not be present in the config file)