Fix maxvalue in DNS plugin

Having a Parameter maxvalue larger that 2G makes Python convert it
to "long" type instead of "int" type. Our framework than fails to
bootstrap the API when it detects long integer in Parameter
maxvalue.

Remove the clashing maxvalue out entirely as we can't transfer
values larger than 2G anyway (xmlrpc limitation).

https://fedorahosted.org/freeipa/ticket/2082

ipalib/plugins/dns.py

@@ -907,7 +907,6 @@ class SIGRecord(DNSRecord):
         Int('original_ttl',
             label=_('Original TTL'),
             minvalue=0,
-            maxvalue=4294967295,
         ),
         Str('signature_expiration',
             _sig_time_validator,