cincinnati: use base_checksum as lookup key

[lucab]

Zincati currently assumes that OS version label is unique and that such label can be used as a lookup key in order to find the current OS node in the update-graph.
While this is likely true in most cases, it may be a bit fragile (across all the FCOS streams) and there is no real reason to do that. Instead, we can directly use the base_checksum for lookups in the graph received from the Cincinnati backend.