Currently it is not possible to use environment variables for the logstash-plugin configuration.

The plugin does not substitute environment variables like ${var}, $var.
It looks like it is a matter of using the "expand(String)" functionality.

http://javadoc.jenkins-ci.org/hudson/EnvVars.html#expand(java.lang.String)

It would be a useful feature to be able to use environment variables within the plugin configuration.