prevent tryRunPluginHelp() execution on non-1st-level commands.

commands comes in the form of `rootCmd 1stCmd 2ndCmd` and the `tryRunPluginHelp()` function takes the command name to run plugin help in the form `docker-<command name>`, if command name matches a plugin name it will display the wrong help message.

to make sure we're using 1stCmd level we check if the root command is the same as the command parent.

e.g. `docker swarm init --help`
```
docker: is the root command
swarm : 1st level command
init  : 2nd level command
--help: is a flag
```

this approach is not taking in consideration if a plugin and 1st level command matches, for example `docker foo` command and `docker-foo` plugin.

Signed-off-by: Hernan Garcia <hernandanielg@gmail.com>
Signed-off-by: Hernan Garcia <hernan.garcia@percona.com>

Author: Hernan Garcia

cmd/docker/docker.go

@@ -137,7 +137,10 @@ func setHelpFunc(dockerCli command.Cli, cmd *cobra.Command) {
 			return
 		}
 
-		if len(args) >= 1 {
+		// commands are chained in the form `rootCmd 1stCmd 2ndCmd...`
+		// if RootCmd and ParentCmd are the same
+		// then my command is 1st level command
+		if len(args) >= 1 && ccmd.Root() == ccmd.Parent() {
 			err := tryRunPluginHelp(dockerCli, ccmd, args)
 			if err == nil {
 				return