config_service: use == for sentinel value
config_service: use == for sentinel value

I forgot to upload this change to
https://codereview.chromium.org/2931673003

TBR=sergeyberezin@chromium.org
BUG=730832

Review-Url: https://codereview.chromium.org/2934783004

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py