Add 'wrk' module

Use it like this:

  make image=wrk
  sudo scripts/run.py -nv -e "wrk -c10 -t2 -d5s http://192.168.122.1:8000/"

It doesn't work on OSv out of the box because of this bug:

  https://github.com/cloudius-systems/osv/issues/411

To make it work you need to apply the workaround mentioned in that link.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>