- allow None in deps_os values (for chromeos buildspecs)
- allow string formatting (%) in expressions (for dart)
- allow empty lists/dicts (for build slaves)
- allow target_os (for internal android)

Bug: 570091, 722206, 725422
Change-Id: I7f3d822a1c4aac16fddd96ef2f9f365e05b2d0e8
Reviewed-on: https://chromium-review.googlesource.com/512724Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Paweł Hajdan Jr. <phajdan.jr@chromium.org>