ClusterSingletonManagerSpec must not use 'node' from other thread, see #3382

* Thank you @rkuhn for finding the reason for the problem
* Sprinkled some warnings
Merge pull request #1479 from akka/wip-3382-ClusterSingletonManagerSpec-patriknw

ClusterSingletonManagerSpec must not use 'node' from other thread, see #3382