rpk: add cobra.NoArg to parent functions.

Cobra won't recognize unknown commands unless
cobra.NoArg is set as an Arg validation parameter
in the command definition.

This means that running 'rpk cluster unknowncommand'
won't return a 'unknown command' error, therefore,
exit as a successful command execution.

Now, cobra will return an 'unknown error' and exit
with code 1.

src/go/rpk/pkg/cli/root.go

@@ -165,6 +165,15 @@ func Execute() {
 		}
 	})
 
+	// Cobra does not return an 'unknown command' error unless cobra.NoArgs is
+	// specified.
+	// See: https://github.com/spf13/cobra/issues/706
+	cobraext.Walk(root, func(c *cobra.Command) {
+		if c.Args == nil && c.HasSubCommands() {
+			c.Args = cobra.NoArgs
+		}
+	})
+
 	cobra.AddTemplateFunc("wrappedLocalFlagUsages", wrappedLocalFlagUsages)
 	cobra.AddTemplateFunc("wrappedGlobalFlagUsages", wrappedGlobalFlagUsages)
 	root.SetUsageTemplate(usageTemplate)