IPA: Add a server mode option
    
    https://fedorahosted.org/sssd/ticket/1993
    
    SSSD needs to know that it is running on an IPA server and should not
    look up trusted users and groups with the help of the extdom plugin
    but do the lookups on its own. For this a new boolean configuration
    option, is introduced which defaults to false but is set to true during
    ipa-server-install or during updates of the FreeIPA server if it is not
    already set.