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>

ipapython/ipautil.py

@@ -852,7 +852,7 @@ def ipa_generate_password(entropy_bits=256, uppercase=1, lowercase=1, digits=1,
     rnd = random.SystemRandom()
 
     todo_entropy = entropy_bits
-    password = ''
+    password = u''
     # Generate required character classes:
     # The order of generated characters is fixed to comply with check in
     # NSS function sftk_newPinCheck() in nss/lib/softoken/fipstokn.c.