Decrease retention of nginx logs to 28 days
Decrease retention of nginx logs to 28 days

Decrease our retention period for nginx logs from 52 to 28 days.

As we serve more traffic, our nginx daily logs increase in size. We're
currently storing 52 days' worth of nginx logs locally. Reduce this to
28 days.

With the current settings, nginx logs are taking up 15GB of disk on the
`cache-?.router.production` nodes:

    mattbostock@production-cache-2:~$ sudo du -h -s /var/log/nginx
    15G     /var/log/nginx

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet