samba_upgradeprovision: detect dns_backend for the reference provision

samba_upgradeprovision: detect dns_backend for the reference provision

If we have a DomainDnsZone partition, we use BIND9_DLZ as backend
and fix errors in the ForestDnsZone and DomainDnsZone partitions.
Note: this should work fine also for SAMBA_INTERNAL.

If the current setup doesn't use dns specific partitions (e.g. alpha13 setups)
we pass dns_backend=BIND9_FLATFILE.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
(cherry picked from commit 58d6d884cf8a8de5a1fa2dfd4a0cbacdff0d2483)