Image download immediately fails when glance returns 500

[Takashi Kajinami]

Description
===========
nova-compute downloads a vm image from glance when launching an instance. It retries requests when it gets 503, but it does not when it gets 500.
When glance uses cinder backend and a image volume is still used (for example because another client is downloading the same image), glance returns 500 and this results in immediate instance creation failure.

Steps to reproduce
==================
* Deploy glance with cinder image store
* Upload an image
* Create an image-boot instance from the image, while downloading the image in background

Expected result
===============
Instance creation succeeeds

Actual result
=============
Instance creation fails because of 500 error from glance

Environment
===========
This has been seen in Puppet OpenStack integration job, which uses RDO master.

Logs & Configs
==============
Example failure can be found in https://zuul.opendev.org/t/openstack/build/fc0e584a70f947d988ac057a8cc991c2

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/nova/+/914761