containers: Add container metadata to CTP request message.

This will be passed down to test_runner to optionally enable running
autoserv through the f20 container images.

BUG=none
TEST=./generate.sh (no errors)

Change-Id: I5c730352899e7cb961d41e1837c0035482de228e
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/3273302
Reviewed-by: Derek Beckett <dbeckett@chromium.org>
Commit-Queue: Derek Beckett <dbeckett@chromium.org>
Auto-Submit: Sean McAllister <smcallis@google.com>
7 files changed