CBQE-1497 add cluster wide stats to plotter.py

In the 90percentil.txt and average.txt, for ns server
stats, we have another key "cluster" in the same hierachy
with those ips like "103110". The values for key cluster is
the ns_server stats for whole cluster.

Change-Id: I99aeb8ffe3a424f6450ec5b12abbf4fa7c3bb656
Reviewed-on: http://review.couchbase.org/27811
Reviewed-by: Tommie McAfee <tommie@couchbase.com>
Tested-by: Chisheng Hong <chisheng@couchbase.com>