MB-12564: only count kv nodes for bucket's summary

I.e. that's count of nodes that's displayed on UI when
creating/editing bucket.

Change-Id: If85386952dc72088c598436c5c89f45ca2db9fe1
Reviewed-on: http://review.couchbase.org/42936
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>