[Fixed JENKINS-8043] "Reload Configuration from Disk" broke swarm clients

When Reloading Configuration from Disk in Jenkins, all swarm clients
were in an unusable state afterwards (eg. missing their labels).
This happened because Swarm Clients were not written into the
configuration file, and got removed when reloading the config.

This patch re-adds all previously existing swarm clients after
reloading the config file.