s4-dns: set TTL value in the NS server part of the SOA record

noticed this when using samba-tool to create a new zone

Signed-off-by: Guenter Kukkukk <kukks@samba.org>

Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>

Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Thu May 30 09:41:20 CEST 2013 on sn-devel-104