use numpy for percentile calculation
use numpy for percentile calculation

Change-Id: Ife776631cf74ed3e045e459552b78428b9f78d2f
Reviewed-on: http://review.couchbase.org/30768
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Pavel Paulau <dd88eded64e90046a680e3a6c0828ceb8fe8a0e7@gmail.com>

couchbase/cbmonitor,couchbase/cbmonitor