Fixes swarm overloading slaves when using the mesos drivers.

The problem is described here:
https://github.com/docker/swarm/issues/1588

This fix also allows reducing the offer timeout which reduces problems with mesos resource consolidation described here:
https://github.com/docker/swarm/issues/1655

Signed-off-by: Guillermo Rodriguez <grodriguez@cmcrc.com>

cluster/mesos/cluster.go

@@ -444,6 +444,7 @@ func (c *Cluster) removeOffer(offer *mesosproto.Offer) bool {
 	found := s.removeOffer(offer.Id.GetValue())
 	if s.empty() {
 		// Disconnect from engine
+		s.engine.Disconnect()
 		delete(c.agents, offer.SlaveId.GetValue())
 	}
 	return found