Add support for notifying a service

[njdancer]

[k3a]

It works great, thanks. I don't understand why this hasn't been merged already.

I would just add this at the beginning of task for iteration body:

			if task.Status.State != "running" {
				continue
			}

And maybe also say which container the signal could not be sent to:

			if err := g.Client.KillContainer(killOpts); err != nil {
-				log.Printf("Error sending signal to container: %s", err)
+				log.Printf("Error sending signal to container %s: %s", container, err)
			}