swarming: improve logging in _ensure_active_slice
swarming: improve logging in _ensure_active_slice

Logging the entire request entity is too verbose.

BUG=950354

Change-Id: I7a97284190f89bf541436e04f42b9d83e86db1d2
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/1558303
Reviewed-by: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>
Commit-Queue: 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