Unshelve with az update the requestSpec.availability_zone this can be inconsistent if schedule fails

[Uggla]

Description
===========
Unshelving a shelved offloaded server will go through the
scheduler to pick a new host, so we update the
RequestSpec.availability_zone here. Note that if scheduling
fails the RequestSpec will remain updated, which is not great.

https://review.opendev.org/c/openstack/nova/+/831507/comment/12fd28cd_42160512/

Steps to reproduce
==================
NA

Expected result
===============
If schedule fails, restore the original az.

Actual result
=============
RequestSpec inconsistent if schedule fails.

Environment
===========
This was introduced in Train (Aug 28, 2019) with the following note:
If we want to change that we need to defer updating the
RequestSpec until conductor which probably means RPC changes to
pass the new_az variable to conductor. This is likely low
priority since the RequestSpec.availability_zone on a shelved
offloaded server does not mean much anyway and clearly the user
is trying to put the server in the target AZ.

Logs & Configs
==============
NA

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/nova/+/911108

[OpenStack Infra (hudson-openstack)]

Related fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/nova/+/914653