jenkins-slave.service: clean up workspace as root

This makes our CI setup robust against test suite jobs that abort in a way they leave files owned by root in the workspace.

One of the branches I've been working on recently does this, and as a result, since a few days, isotesters are regularly left offline because jenkins-slave.service fails to start, because it is not allowed to "/bin/rm -rf /var/lib/jenkins/workspace/" as the "jenkins" user. Every time, I have to manually re-run this command as root, then restart jenkins-slave.service. Let's automate this :)