install: improve CLI positional argument handling
    
    Instead of specifying which knobs should be positional arguments in
    cli.install_tool(), do it using a flag in knob definition, where the rest
    of CLI configuration is.
    
    As a side effect, the usage string for CLI tools can now be generated
    automatically.
    
    https://fedorahosted.org/freeipa/ticket/6392
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>