Added functionality to remove old backup files.

* The TTL for backup files is hardcoded to 90 days.
* For cleanup the script 'wipe-tmp' from the same project is required,
  otherwise a warning will be issued.
* Changed exit code from 2 to 0 if the backup file already exists locally
  because this can be considered to be a normal situation depending on the
  number of times this script is being run per day.