change the slave configuration, so that the nodes are brought online only on demand

This will:
 * reduce the amount of used memory: currently any given slave.jar
   process takes something between 200 and 350 MB of resident memory,
   which easily goes over the 10 GB mark…
 * in general, reduce the number of running processes on the master,
   which can't possibly be a bad thing
 * make possible to not even start jobs on the slaves if a host goes offline
   for whatever reason
At the same time, starting slave.jar is not such an expensive process,
and the delay in starting the jobs from an offline node is totally
negligible

Signed-off-by: Mattia Rizzolo <mattia@debian.org>