cmd/coordinator: push new logging down into the buildlet creation

Next step is logging it to datastore/bigquery. Then stats and
visualizing with trace viewer later.

Updates golang/go#12669

Change-Id: I039097791993dbe3d4aceb18d2a08db8299f3c05
Reviewed-on: https://go-review.googlesource.com/21640
Reviewed-by: Andrew Gerrand <adg@golang.org>
5 files changed