MB-1143: enable adding 2.5 nodes to 2.0 clusters

Change-Id: If58bc3a30a6b3a7ec6583af9806a0b320169e63a
Reviewed-on: http://review.couchbase.org/29880
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>