Add --repeat flag to run_on_bots.py
Add --repeat flag to run_on_bots.py

This permits creating super easy load tests. This is to help reproduce bug 89 on
the canary infrastructure.

R=csharp@chromium.org
BUG=

Review URL: https://codereview.appspot.com/76100044

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py