Precallback validator was failing when a zone-relative name was
used as a NS record (for example record "ns" in a zone "example.com").
However, this is valid in BIND and we should allow it as well.
Imports in dns module had to be switched to absolute imports
(available from Python 2.5) to deal with a conflict of IPA dns
module and dnspython module.
https://fedorahosted.org/freeipa/ticket/2630