DNS Locations: add index for ipalocation attribute
    
    For performace ipalocation should be indexed because it is used by
    referint plugin
    
    https://fedorahosted.org/freeipa/ticket/2008
    
    Reviewed-By: Petr Spacek <pspacek@redhat.com>
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>