Don't permit node addition to cluster when node is already part of cluster and is not failed over

Data can be lost if the following happens:

1. In a 3 node cluster (A, B, and C), C goes down
2. C loses its data and config and comes back up with the same name (on the same host)
3. C is added back to cluster and rebalance is performed

C will resume its old position in the cluster (though there will be config merge conflicts) and will pick up managing the active vbuckets it used to have. These vbuckets are now empty of course, so data is lost.