MB-9096: allow joining 2.x clusters

Change-Id: I9fd9690bb8c914d34e84d134d6e328da9e027de4
Reviewed-on: http://review.couchbase.org/29325
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>