[Reviewer Andy] Reverse sense of merged attributed to a joining attribute to make upgrades of existing systems work

Andy

Can you review please.  I've tested by

-  spinning up a new deployment from scratch and checking that the cluster_settings files is set up as expected despite there being no attributes set
-  growing the sprout cluster to check that the new node has the joining attribute set and the cluster_settings are right
-  finishing the scale up to check that the attributes are cleared and the cluster_settings change to the stable setting.

I ran live tests at each stage to check sprout was doing the right thing.

Mike