[isolated] Silence output when -quiet is true
[isolated] Silence output when -quiet is true

Bug: 1099655
Change-Id: Ibfc679403d5e07385566d35267ae7c7f3a1281c4
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2325931
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Commit-Queue: Ye Kuang <1cd89f22f3967fc3dcb883b53940be4b98ec483f@google.com>
Auto-Submit: Ye Kuang <1cd89f22f3967fc3dcb883b53940be4b98ec483f@google.com>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go