MB-8129:  cbworkloadgen is in bin, not bin/tools

Change-Id: Ic46ffa5887398f2b385c77198c2b6918733e0a06
Reviewed-on: http://review.couchbase.org/25835
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>