[swarming] Less logspam in cron_update_bot_info.
[swarming] Less logspam in cron_update_bot_info.

We have ~25000 bots. Spamming 500 log lines is a bit excessive.
Spam only 50.

R=40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org

Change-Id: I99de05c5a8a3e36f1d6bb1f9a5b741c38424949a
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/3821164
Reviewed-by: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

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