DNS Locations: dnsservers: add required objectclasses
    
    Objectclass: idnsServerConfigObject - stores configuration values for
    DNS servers
    
    Attributetype: idnsServerId - identifier of dns server (server hostname)
    
    https://fedorahosted.org/freeipa/ticket/2008
    
    Reviewed-By: Petr Spacek <pspacek@redhat.com>
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>