[swarming] Add -cas-instance option to swarming trigger command
[swarming] Add -cas-instance option to swarming trigger command

This will be used by local_smoke_test.py to pass dummy instance.
Otherwise, it fails at L417 with local Swarming server address.

Bug: 1135017, 1255535
Change-Id: I2a232f005730cfb4fd2753bde15d75c724e0b520
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3267569
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Auto-Submit: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Reviewed-by: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

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