Make Rally Resilient to Downscaling

Right now the rally logic works ok for scaling up.  However, if a user manually deletes a node, or if they scale down the IGM and the node is deleted, then new nodes will attempt to rally on a node that no longer exists.

The relevant code is here:
https://github.com/couchbase-partners/google-deployment-manager-couchbase/blob/master/simple/server.sh#L71-L115