Parallelizing via Free Style Jobs Fails

When I parallelize via the free style jobs no exclusion files are created.
This leads to all tests running on all nodes.

I can reproduce this behaviour both on my private Jenkins instance and using the demo.

Steps to reproduce:
* Install the demo, following the docker steps described [here](https://github.com/jenkinsci/parallel-test-executor-plugin/blob/master/demo/README.md)
* Run the _main_
* After _main_ finishes run it again.

Expected results:
* The second time _main_ is started its 400 tests are split amongst executors, reducing the total execution time.

Actual result:
* All tests were run on the 5 started executors, leading to 2000 tests run in total.