py3: dogtag.py: fix bytes warnings
    
     /usr/lib/python3.5/site-packages/ipaserver/plugins/dogtag.py:1438: BytesWarning: str() on a bytes instance
       "parse_result:\\n%s" % (parse_func.__name__, xml_text, result))
    
    https://pagure.io/freeipa/issue/4985
    
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>