DNS: Fix upgrade - master to forward zone transformation

This happens when upgrading from IPA <= 4.0 to versions 4.3+.

DNS caching might cause false positive in code which replaces master zone
with forward zone. This will effectivelly delete the master zone
without adding a replacement forward zone.

Solution is to use skip_overlap_check option for dnsforwardzone_add command
so zone existence check is skipped and the upgrade can proceed.

https://fedorahosted.org/freeipa/ticket/5851

Reviewed-By: Martin Basti <mbasti@redhat.com>

ipaserver/install/plugins/dns.py

@@ -302,7 +302,9 @@ class update_master_to_dnsforwardzones(Updater):
                 try:
                     kw = {
                         'idnsforwarders': zone.get('idnsforwarders', []),
-                        'idnsforwardpolicy': zone.get('idnsforwardpolicy', [u'first'])[0]
+                        'idnsforwardpolicy': zone.get('idnsforwardpolicy',
+                                                      [u'first'])[0],
+                        'skip_overlap_check': True,
                     }
                     self.api.Command['dnsforwardzone_add'](zone['idnsname'][0], **kw)
                 except Exception as e: