[nova][ptg] Support re-configure deleted_on_termination in server

Spec is https://review.opendev.org/#/c/580336/ Most people seem to think this makes sense but realize there are already other ways to do this (snapshot) and therefore it's not totally necessary.

The agreement in the room was to post the code up for the change, as this will help sell people on it if it's trivial enough and document the use case (i.e. are there scenarios where this would make life 10x easier?)