add support for log formats

I want to be able to define new log formats in the nginx configuration as follows:

```
override['nginx']['log_formats'] = {
  "acc_log" => %{'$remote_addr - $remote_user [$time_local] '
                   '"$request" $status $bytes_sent '
                   '"$http_referer" "$http_user_agent" "$gzip_ratio" '
                   '"$sent_http_x_cache" $request_time'}
}
```

I've been trying to follow your contributing guidelines, I signed your ICLA, but JIRA says that the issue tracker has moved to github, so I am not sure if I'm doing it right.