celery: link test less often
celery: link test less often

This is causing the disk of the CBCs to fill up quickly, and their
logrotate seems a bit flakey

Reducing the rate will ensure the disks fill up less often

Signed-off-by: Toby Lorne <527052641d65eef236eeef9545b2acac74c35d57@digital.cabinet-office.gov.uk>

alphagov/notifications-api,alphagov/notifications-api