Comment 2 for bug 1875287

The following options are available:
1. Add image auto signature for any image Nova puts to Glance.
2. Disable image signature validation for specific operations like unshelve.
3. Make signature for images created by nova manually or e.g. cron script (there could be a race).

For 1. there is a PoC: https://review.opendev.org/723320