ostree-image: accept only signed updates (cherry-picked from 2022dev1)

Starting version v2020.7 libostree upstream provides deltas with signed metadata allowing to validate the creator of the delta file itself, not only the commit inside.

The new repository option core.sign-verify-deltas=true forces both libostree and AUM to accept deltas with signed metadata only.

This change affect only to systems using libostree v2020.7+. Apertis Update Manager must be updated to accept deltas with signed metadata.

Signed-off-by: Denis Pynkin denis.pynkin@collabora.com