MB-8163: don't attempt to track stats if not all nodes are 2.0.2

Because it'll clearly fail.

Change-Id: Idae4b7f6d721191f3837cbf7589c7624aa0d03b0
Reviewed-on: http://review.couchbase.org/26000
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>