fn: memory limit adjustments (#746)

1) limit kernel memory which was previously unlimited, using
   same limits as user memory for a unified approach.
2) disable swap memory for containers

api/agent/drivers/docker/docker.go

@@ -137,6 +137,8 @@ func (drv *DockerDriver) Prepare(ctx context.Context, task drivers.ContainerTask
 			Env:          envvars,
 			Cmd:          cmd,
 			Memory:       int64(task.Memory()),
+			MemorySwap:   int64(task.Memory()), // disables swap
+			KernelMemory: int64(task.Memory()),
 			CPUShares:    drv.conf.CPUShares,
 			Hostname:     drv.hostname,
 			Image:        task.Image(),