MB-8400: only randomize nodes list for actual bucket infos

Change-Id: I0b92f8c8807a65b3918f35ac96739ec19cf06d2b
Reviewed-on: http://review.couchbase.org/28359
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>