samba_dnsupdate: Do not rewrite krb5.conf in selftest

samba_dnsupdate: Do not rewrite krb5.conf in selftest

The samba_dnsupdate script is responsible to provision the DNS entries.
The private krb5.conf uses dns lookups to find the KDC to acquire a
Kerberos ticket.  Obviously this will fail because currently we are are
in the process of adding the DNS entries for the KDC.

If we are inside of selftest we need to use the krb5.conf created by
selftest itself.

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Andrew Bartlet <abartlet@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>