Handle rotation of JSON log files

We were not managing the /etc/logrotate.d/nginx file, so
were relying on the package default. This wasn't aware of our *.json
log files. Oops!
I think this is the same as the logrotate file shipped by the nginx package with the exception of adding in *.json.

That said there *is* a rotate option to the init script so, yes, lets use it. (Means we only have to update the pid file in two places, not three etc.)