Support usage as plugin for tools like kubectl (#2018)

In this case the executable is `kubectl-plugin`, but we run it as:

    kubectl plugin

And the help text should reflect the actual usage of the command.

To create a plugin, add the cobra.CommandDisplayNameAnnotation:

    rootCmd := &cobra.Command{
        Use: "plugin",
        Annotations: map[string]string{
            cobra.CommandDisplayNameAnnotation: "kubectl plugin",
        }
    }

Internally this change modifies CommandPath() for the root command to
return the command display name instead of the command name. This is
used for error messages, help text generation, and completions.

CommandPath() is expected to have spaces and code using it already
handle spaces (e.g replacing with _), so hopefully this does not break
anything.

Fixes: #2017

Signed-off-by: Nir Soffer <nsoffer@redhat.com>