Java exception when trying to run plugin commands when jmx monitoring is enabled.

- Version: tested on 5.2.1 & 5.3
- Operating System: Any
- Config File (if you have sensitive info, please remove it):
jvm.options file includes :

`-Dcom.sun.management.jmxremote
-Dcom.sun.management.jmxremote.port=9010
-Dcom.sun.management.jmxremote.local.only=false
-Dcom.sun.management.jmxremote.authenticate=false
-Dcom.sun.management.jmxremote.ssl=false
-Djava.rmi.server.hostname=localhost `

- Steps to Reproduce:

Add above lines to jvm.options file and start logstash with a simple stdin and stdout conf file.
Logstash will start successfully , then try to install a plugin for example:
`$ bin/logstash-plugin install x-pack`

Which will result in :
`Error: Exception thrown by the agent : java.rmi.server.ExportException: Port already in use: 9010; nested exception is:
java.net.BindException: Address already in use
`
Any plugin command such as list or remove will result in the same. A similar issue existed in old version https://github.com/elastic/logstash/issues/3247 however with the introduction of jvm.options file in recent versions this file should be the best way for this configuration.