Add gidnumber minvalue

Do not accept invalid GID values in IPA user/group plugins.

https://fedorahosted.org/freeipa/ticket/2335

ipalib/plugins/group.py

@@ -119,6 +119,7 @@ class group(LDAPObject):
             cli_name='gid',
             label=_('GID'),
             doc=_('GID (use this option to set it manually)'),
+            minvalue=1,
         ),
     )