[filebeat] Add convert_timezone to nginx module

[pc-jedi]

Currently the nginx module does not have a convert_timezone configuration option.
Since Kibana assumes a timestamp field to be stored as UTC in ES, you will see your logs having a wrong timestamp.
This would not be a big issue.
But if you have configured your server timezone to UTC+1 and insert a nginx log with [02/Jan/2019:10:00:00 +0100], which is 9AM UTC, the @timestamp will be 10AM UTC.
If you now want to see your log in Kibana this will not be possible, because queries in Kibana are limited by the current timestamp. Therefore you have to wait util it is 10AM UTC to be able to see your log in Kibana.

The easiest workaround for that would be setting your server timezone to UTC, which I don't like.

It would be nice to have something like the system module convert_timezone.

Since there are also some more module, which don't have this option it would be the best to add this to all modules.

[kaiyan-sheng]

This seems to be a subtask for #9993

[pc-jedi]

@kaiyan-sheng I think you made a small mistake.
https://github.com/elastic/beats/blob/master/filebeat/module/nginx/access/ingest/default.json#L77
I think the field must be beat.timezone and not ``event.timezone` according to the docu

Same here:
https://github.com/elastic/beats/blob/master/filebeat/module/nginx/error/ingest/pipeline.json#L21