Publish clean state when joining (PublishStart), see #2871

* The failure in JoinTwoClustersSpec was due to missing publishing
  of cluster events when clearing current state when joining
* This fix is in the right direction, but joining clusters like this
  will need some design thought, creating ticket 2873 for that
Merge pull request #1000 from akka/wip-2871-publish-on-clearState-patriknw

Publish clean state when joining (PublishStart), see #2871