attestation: Support default and test Verified Access servers

BUG=b:69687094
TEST=unit tests

Change-Id: I9b4587d155099d9e5f94bfe0a881f73e2acf1a82
Reviewed-on: https://chromium-review.googlesource.com/795216
Commit-Ready: Yves Arrouye <drcrash@chromium.org>
Tested-by: Yves Arrouye <drcrash@chromium.org>
Reviewed-by: Andrey Pronin <apronin@chromium.org>