I could not upgrade the CI Jenkins to 2.319.1 since I had to fix the Jenkins Gearman plugin and publish a new release of it. It is done now T298949
It is too late (11pm) to attempt an upgrade to 2.319.1. I will upgrade directly to the security update 2.319.2 when it is published and upgrade the Gearman plugin.
The master node got renamed to (built-in) https://integration.wikimedia.org/ci/computer/(built-in)/ builds are running fine on it. I then upgraded the Gearman plugin and all jobs seem to be working including the workflow ones.
Actually the migration for master to built-in has to be applied via the Web UI. There are requisites and guidances listed at http://www.jenkins.io/doc/book/managing/built-in-node-migration/