MB-9714: use bucket's nodes to get count for storage totals

Change-Id: I3241c6079b57eccb896053cc1b4453384fec2040
Reviewed-on: http://review.couchbase.org/31030
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>