suite_scheduler: Add an option to skip all variants

Some bugs filed by users recently highlighted this need.
If skip_variants = False, they can specify exact variants to run.

BUG=chromium:1113790
TEST=./gen

Change-Id: I1a56025da02e125625d7882c5928c729827669c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/2375966
Commit-Queue: Dhanya Ganesh <dhanyaganesh@chromium.org>
Tested-by: Dhanya Ganesh <dhanyaganesh@chromium.org>
Auto-Submit: Dhanya Ganesh <dhanyaganesh@chromium.org>
Reviewed-by: Sean Abraham <seanabraham@chromium.org>
2 files changed