ostree-image: accept only signed updates

Starting version v2020.7 `libostree` upstream provides deltas with
[signed metadata](https://github.com/ostreedev/ostree/pull/1985)
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](pkg/apertis-update-manager!57

)
to accept deltas with signed metadata.

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