crontab: add periodic cleanup the collectd RRD (graphs)

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>