fixes #809 - Flaky test DeploymentActorTest.RestartApp

Removed the global `ActorSystem` to eliminate the possibility of events leaking between tests. In the end it was a wrong app version in one of the `TASK_KILLED` events that created a race in the `StartingBehavior`. Depending on whether or not the `TaskStartActor` was already subscribed to the event bus when this event was published, the test either failed or succeeded.