Move "stress" test generation script to QA repo

Move "stress" test generation script to QA repo

Move the util/generate-stress-test-configs.py script from the BIND 9
source repository to the BIND 9 QA repository.  This simplifies the
maintenance of that script by eliminating the need to backport every
change applied to it to multiple branches.

(cherry picked from commit 4f7f420534fc939107654a39ab94aa8d4e9d9c45)