py3: cainstance: replace mkstemp with NamedTemporaryFile
With Python3 files must be opened in textual mode to write text, and
best practise is to use fileobject instead fo os.write() and manual
encodig
https://fedorahosted.org/freeipa/ticket/4985
Reviewed-By: Christian Heimes <cheimes@redhat.com>
Reviewed-By: Jan Cholasta <jcholast@redhat.com>