swarming: retries 3 times when call run_parent
swarming: retries 3 times when call run_parent

run_parent tends to be failed, let me increase retries from default 1.
https://chromium.googlesource.com/infra/luci/luci-py/+/2f77519204c6b3e48e3f30ace670ff48cebe5d65/appengine/components/components/datastore_utils/txn.py#48

Bug: 1018982
Change-Id: I2727f95a8a72251e87aff0d1c827ebfea548064d
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/1886088
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Reviewed-by: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

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