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!
For this PR, I'd rather keep it the same as the package default. Have checked 14.04 nginx package though, and the logrotate.d file stays the same.

Also stays managed by the init.d script though, which was initially my reason for thinking 'hey maybe we shouldn't change this'.