Config test should not try to bind to jmx port

We [fixed this previously in 2.x](https://github.com/elastic/logstash/pull/4759), but configtest is back to trying to bind to jmx when the jmx settings are specified in jvm.options on 5.x (regression?)

As a result, if you already have LS running with jmx settings, and then try to run the same pipeline but using configtest (-t) parameter, the configtest will fail:

```
Error: Exception thrown by the agent : java.rmi.server.ExportException: Port already in use: 9994; nested exception is:
java.net.BindException: Address already in use
```