Add argument help to CLI
    
    CLI command help contains a documentation for all options that can
    be passed to commands. However, help strings for positional
    arguments are not included.
    
    This patch uses an OptionParser description field to list all
    command arguments as OptionParser does not have a native support
    to provide such information to user.
    
    https://fedorahosted.org/freeipa/ticket/1974