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)
Upstream-commit: 06bc4e07529a93abede35ea8fcc2906c5963f2a3
Component: engine

components/engine/docs/sources/reference/commandline/cli.md

@@ -732,6 +732,15 @@ specify this by adding the server name.
     example:
     $ docker login localhost:8080
 
+## logout
+
+    Usage: docker logout [SERVER]
+
+    Log the user out of a docker registry server, if no server is specified "https://index.docker.io/v1/" is the default.
+
+    example:
+    $ docker logout localhost:8080
+
 ## logs
 
     Usage: docker logs CONTAINER