CBQE-1340 add plotter.py to gather pdf reports from cbmonitor

The script do 2 things. Insert phase timestamp into cbmonitor as
plotting snapshot reference and download those pdf reports to your
local folder. Also dump 90 percentile value, avg value for all the
stats in each phase into txt files in json format.

Change-Id: I8b4ba39892d4cd578d8216af1245c9cf3e4cc8d5
Reviewed-on: http://review.couchbase.org/27239
Reviewed-by: Chisheng Hong <chisheng@couchbase.com>
Tested-by: Chisheng Hong <chisheng@couchbase.com>