crontab: add periodic cleanup the collectd RRD (graphs)
- Created (mostly) for old openvpn graphs
- RRD removed when no graph modification for +365 days
- chosen since graph max out is 365 days
- fcron job runs once per week
- chosen since this is just a cleanup and it doesnt need to run everyday
Note: logging can be added if needed.
Signed-off-by: Jon Murphy <jon.murphy@ipfire.org> Reviewed-by: Bernhard Bitsch <bbitsch@ipfire.org> Reviewed-by: Michael Tremer <michael.tremer@ipfire.org>