CBQE-1356 automate cbworkloadgen tool test

Change-Id: I302f3edf34dcdda45731b6e9af50a4fc112ebd5b
Reviewed-on: http://review.couchbase.org/27370
Reviewed-by: Chisheng Hong <chisheng@couchbase.com>
Tested-by: Chisheng Hong <chisheng@couchbase.com>