[swarming] create output directory if not exists
[swarming] create output directory if not exists

I think it is useful if directory is created automatically.

Change-Id: Ie688d0113b9e752dfede001a48bc329ba81b2d04
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2428609
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

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