dns plugin: Fix zone normalization under Python 3

In Python 3, str.encode('ascii') converts to bytes, and str()
(nicknamed unicode() in IPA) returns the string representation
of an object, which is b'...' for bytes.

So, unicode('...'.encode('ascii')) results in "b'...'".

Change the code to only call encode() for the error.

Part of the work for https://fedorahosted.org/freeipa/ticket/4985

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

ipalib/plugins/dns.py

@@ -1755,9 +1755,11 @@ def _normalize_zone(zone):
     if isinstance(zone, unicode):
         # normalize only non-IDNA zones
         try:
-            return unicode(zone.encode('ascii')).lower()
+            zone.encode('ascii')
         except UnicodeError:
             pass
+        else:
+            return zone.lower()
     return zone