tools/run_on_bots: Enable giving a hash instead of script.
tools/run_on_bots: Enable giving a hash instead of script.

This allows you to specify an isolated hash instead of giving a script.
This makes it easier to test an existing uploaded test.

(Also fix description for extra arguments.)

Change-Id: Ifdf3b720adee9ba62f46d7729baaf10695d5f27a
Reviewed-on: https://chromium-review.googlesource.com/641733
Commit-Queue: Tim 'mithro' Ansell <f857bf86f3aed4a1021a5247bd2399c65b7ad5cd@chromium.org>
Reviewed-by: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>

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