dns: move code shared by client and server to separate module

Move the shared code to a new ipalib.dns module.

https://fedorahosted.org/freeipa/ticket/4739

Reviewed-By: David Kupka <dkupka@redhat.com>

ipalib/util.py

@@ -21,6 +21,8 @@
 Various utility functions.
 """
 
+from __future__ import absolute_import
+
 import os
 import socket
 import re