Use log methods instead of $log
Use log methods instead of $log

Because using `log` makes users to be able to specify log level in each plugin.

After applying this patch, users can write `@log_level` config syntax in this
plugin:

```
<source>
  type elb_log
 ...
 @log_level warn
</source>
```

shinsaka/fluent-plugin-elb-log