Fix incorrect error message occurence when re-adding the trust

You cannot re-add the trust and modify the range in the process.
The check in the code was malfunctioning since it assumed that
range_size parameter has default value. However, default value
is assigned only later in the add_range function.

https://fedorahosted.org/freeipa/ticket/3870

ipalib/plugins/trust.py

@@ -423,7 +423,7 @@ sides.
                     ))
 
         base_id = options.get('base_id')
-        range_size = options.get('range_size') != DEFAULT_RANGE_SIZE
+        range_size = options.get('range_size')
 
         if old_range and (base_id or range_size):
             raise errors.ValidationError(