Fix #5986 - Override default field order

It is sometimes necessary to alter the order in which fields are rendered, or remove individual fields, especially when subclassing forms.
With this patch one can easily override the default order with a list of field names.

Ticket https://code.djangoproject.com/ticket/23936
It would be good to add a test for this(probably can go in `django.contrib.auth.tests.test_forms.PasswordChangeFormTest`).