DLQ enabled for x-pack internal indices

*Original comment by @ppf2:*

5.5.0

In the Logstash DLQ data folder, there is a folder automatically created for the logstash monitoring index once x-pack monitoring is enabled. Even though it isn't dropping any events into the DLQ, the user finds it strange that we are enabling DLQ for internal indices and they would like some way to turn this off.

```
ls -lah .monitoring-logstash/
total 4.0K
drwxr-xr-x 2 logstash logstash 32 Aug 25 01:57 .
drwxr-xr-x 4 logstash logstash 46 Aug 25 01:57 ..
-rw-r--r-- 1 logstash logstash 1 Aug 25 01:57 1.log
-rw-r--r-- 1 logstash logstash 0 Aug 25 01:57 .lock
```