improve computation of Job name input-hash suffix

This is an improvement to avoid recreating the sync-openstack-images Job if not needed; this is done by ensuring that the hash we add at the en of the Job name changes only when relevant, and in particular does not change if the list of OS images given as input to the chart does not change which images are synced.

The checksum value is removed (it wasn't even useful before this MR, and now even less, if we can say so).

This MR is integrated and tested in sylva-core MR sylva-projects/sylva-core!4097 (merged)