support multiple plugin paths

[cnf]

Hi,

If one uses the docker image for graylog, the plugins live in /usr/share/graylog/plugin. In order to add new ones, one can bind a host dir to the container docker run -v /host/graylog/plugins:/usr/share/graylog/plugin, but then you overwrite the supplied plugins.

One could copy the supplied plugins to the host dir, but then they would not be upgraded when a new version of graylog image is run.

proposed solution: If graylog can scan its plugin dir recursively, then one can bind mount the 3rd party plugins into the existing dir, and keep the supplied ones through upgrades: docker run -v /host/graylog/plugins:/usr/share/graylog/plugin/3rdparty

This would create a layout like this, for example:

/usr/share/graylog/plugin
|-- graylog-plugin-beats-1.1.1.jar
|-- graylog-plugin-collector-1.1.1.jar
|-- graylog-plugin-enterprise-integration-1.1.1.jar
|-- graylog-plugin-map-widget-1.1.1.jar
|-- graylog-plugin-pipeline-processor-1.1.1.jar
|-- usage-statistics-2.1.1.jar
`-- 3rdparty
    |-- graylog-plugin-netflow-0.1.1.jar
    |-- graylog-plugin-slack-2.2.1.jar
    |-- graylog2-plugin-input-httpmonitor-1.0.3.jar

The main plugin dir holds the included plugins, the 3rd party one the externally linked ones.

[bernd]

Duplicate of https://github.com/Graylog2/graylog2-server/issues/6482