Solve UnreachableNodeJoinsAgainSpec problem, see #3247

* UnreachableNodeJoinsAgain failed because of gated connection
* Removed default test value of retry-gate-closed-for, instead
  default from reference.conf is used, i.e. 0s
* deadLetters logging love
Merge pull request #1361 from akka/wip-3247-UnreachableNodeJoinsAgainSpec-patriknw

Solve UnreachableNodeJoinsAgainSpec problem, see #3247