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)
fix doc references

Docker-DCO-1.1-Signed-off-by: Daniel, Dao Quang Minh <dqminh89@gmail.com> (github: dqminh)
update style

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
Merge pull request #7106 from vieux/logout

Add logout command