Limit max username length to 255 in config-mod

This value is same as is used in user and stageuser commands.

https://fedorahosted.org/freeipa/ticket/5774

Reviewed-By: David Kupka <dkupka@redhat.com>

ipalib/plugins/config.py

@@ -128,6 +128,7 @@ class config(LDAPObject):
             cli_name='maxusername',
             label=_('Maximum username length'),
             minvalue=1,
+            maxvalue=255,
         ),
         IA5Str('ipahomesrootdir',
             cli_name='homedirectory',