CBQE-2300: vbuckettool: define vbuckets to load

Change-Id: I1f2847ace1c67312d123652dd2bb5d3f56f31bf0
Reviewed-on: http://review.couchbase.org/38829
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>