Use /usr/bin/python2

Part of the effort to port FreeIPA to Arch Linux,
where Python 3 is the default.

FreeIPA hasn't been ported to Python 3, so the code must be modified to
run /usr/bin/python2

https://fedorahosted.org/freeipa/ticket/3438

Updated by pviktori@redhat.com

ipaserver/install/ipa_server_certinstall.py

@@ -1,4 +1,3 @@
-#! /usr/bin/python
 # Authors: Karl MacMillan <kmacmillan@mentalrootkit.com>
 #          Jan Cholasta <jcholast@redhat.com>
 #