Add logrotate rule to nginx JSON logs

**This is not ready to merge. I don't know what I'm doing so this pull request is more to get people to remember to do something with these files.**

Nginx JSON access and error logs are at about 2GB in size on the jumpbox.

```
root@jumpbox-1:/var/log/nginx# ls -alh | grep G
total 3.1G
-rw-r--r--  1 www-data root 1.9G Mar 24 15:46 alerts.production.performance.service.gov.uk.access.log.json
-rw-r--r--  1 www-data root 1.2G Mar 24 15:46 graphite.production.performance.service.gov.uk.access.log.json
```