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

Hi, nova: We would like to have a spec freeze exception for the spec: Support re-configure deleted_on_termination in server [1], and it’s PoC code in [2] I will attend the nova meeting on February 20 2020 1400 UTC as much as possible. [1] https://review.opendev.org/#/c/580336/ [2] https://review.opendev.org/#/c/693828/ brinzhang