Plugin doesn't work if port 9650 not opened - 1.9.0

Hello,

Since 1.9.0 version, root cause analysis is communicating with performance analyser through port 9650. If this port is not open, the plugin crash. Anywhere in your documentation you mentioned this port. Users who not having this port opened by default will face the crash.

Issue on github : https://github.com/opendistro-for-elasticsearch/performance-analyzer/issues/205

Thanks,
Julien Elk