CBQE-1004: template support various doc size

example:  python cbsystest.py ....   --size 128 256 512

Change-Id: I4a9df0332bcd6f60d17f947c4b807a7915f1237b
Reviewed-on: http://review.couchbase.org/24333
Reviewed-by: Ketaki Gangal <ketakigangal@gmail.com>
Tested-by: buildbot <build@couchbase.com>