Bug 32093: Make help and code match for bg-days in cleanup_database.pl

Bug 32093: Make help and code match for bg-days in cleanup_database.pl

There as a mismatch between the parameter name in documentation (bg-days)
and what was checked in the code (bg-jobs). This makes both match.

To test:
* Apply patch
* Make sure you have some background_jobs older than one day
* Verify the help of the script documents the parameter bg-days
* Run the cleanup_database.pl job
  Example:  ./misc/cronjobs/cleanup_database.pl --bg-days  --confirm -v
* Verify there are no errors and the lines have been deleted as expected

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>