DNS install check: allow overlapping zone to be from the master itself

When re-running `ipa-server-install --setup-dns` on already installed
server, we do not get to the check of being already installed because
DNS zone overlap forces us to fail earlier.

Change exception returned for this case from check_zone_overlap() to
return structured information that allows to understand whether we are
finding a conflict with ourselves.

Use the returned information to only fail DNS check at this point if DNS
zone overlap is generated by a different name server than ourselves.

Reviewed-By: Christian Heimes <cheimes@redhat.com>

ipapython/dnsutil.py

@@ -39,6 +39,12 @@ if six.PY3:
 logger = logging.getLogger(__name__)
 
 
+class DNSZoneAlreadyExists(dns.exception.DNSException):
+    supp_kwargs = {'zone', 'ns'}
+    fmt = (u"DNS zone {zone} already exists in DNS "
+           "and is handled by server(s): {ns}")
+
+
 @six.python_2_unicode_compatible
 class DNSName(dns.name.Name):
     labels = None  # make pylint happy
@@ -374,10 +380,7 @@ def check_zone_overlap(zone, raise_on_error=True):
                          zone, e)
             ns = []
 
-        msg = u"DNS zone {0} already exists in DNS".format(zone)
-        if ns:
-            msg += u" and is handled by server(s): {0}".format(', '.join(ns))
-        raise ValueError(msg)
+        raise DNSZoneAlreadyExists(zone=zone.to_text(), ns=ns)
 
 
 def _mix_weight(records):