Ignore unknown arguments on the top-level command.

This allows passing argument to plugins, otherwise they are caught by the parse
loop, since cobra does not know about each plugin at this stage (to avoid
having to always scan for all plugins) this means that e.g. `docker plugin
--foo` would accumulate `plugin` as an arg to the `docker` command, then choke
on the unknown `--foo`.

This allows unknown global args only, unknown arguments on subcommands (e.g.
`docker ps --foo`) are still correctly caught.

Add an e2e test covering this case.

Signed-off-by: Ian Campbell <ijc@docker.com>

cli-plugins/examples/helloworld/main.go

@@ -33,6 +33,7 @@ func main() {
 			},
 		}
 
+		var who string
 		cmd := &cobra.Command{
 			Use:   "helloworld",
 			Short: "A basic Hello World plugin for tests",
@@ -41,9 +42,11 @@ func main() {
 			// hook.
 			PersistentPreRunE: plugin.PersistentPreRunE,
 			Run: func(cmd *cobra.Command, args []string) {
-				fmt.Fprintln(dockerCli.Out(), "Hello World!")
+				fmt.Fprintf(dockerCli.Out(), "Hello %s!\n", who)
 			},
 		}
+		flags := cmd.Flags()
+		flags.StringVar(&who, "who", "World", "Who are we addressing?")
 
 		cmd.AddCommand(goodbye, apiversion)
 		return cmd