MB-3726 enable aggregated server stats for all nodes

if "All Server Nodes" is selected the UI will show aggregated
server stats. aggregation method is SUM for all stats except
CPU utilization % for which it is MAX

I also fixed unrelated problem of server stats not being
available in "show by server" view

Change-Id: I959d57f8ce34c220995dd887e53425574ae78491
Reviewed-on: http://review.couchbase.org/28960
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>