docs/scripts: adapt to change in wrk2 code management

- Add make target for traffic generator tools, e.g., wrk2.
- Update SSL reverse proxy user guide and test script to use wrk2
  built via repo manifest method.

Signed-off-by: Liangxing Wang <liangxing.wang@arm.com>
Reviewed-by: Nathan Brown <nathan.brown@arm.com>
Change-Id: I480c076b0b3c3bc0a18937b18e8e9e037274cf9e