swarming: add cron entry for external scheduler cancellations
swarming: add cron entry for external scheduler cancellations

BUG=chromium:898323

Change-Id: Ib51a351c45f926586a1ee96ef4c631c1b95af0fe
Reviewed-on: https://chromium-review.googlesource.com/c/1354520
Reviewed-by: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>
Commit-Queue: Aviv Keshet <178e8ae417efbe404feca7ab4cb8f0ba8c109369@chromium.org>

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