Avoid multiple client discovery with fixed server list
    
    In client discovery module, we used to run up to three discovery
    processes even though we received a fixed list of servers to connect
    to. This could result in up to 3 identical "not an IPA server" error
    messages when the passed server is not an IPA server.
    
    Error out immediately when we are discovering against a fixed set
    of servers.
    
    Related to fixes in https://fedorahosted.org/freeipa/ticket/3418