py3: service.py: replace mkstemp by NamedTemporaryFile
    
    NamedTemporaryfile can be used in more pythonic way and file can be
    opened in textual mode that is required with PY3
    
    https://fedorahosted.org/freeipa/ticket/4985
    
    Reviewed-By: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>