Set default value for -service-account-json flag.
Set default value for -service-account-json flag.

BUG=699296
R=nodir@chromium.org

Review-Url: https://codereview.chromium.org/2732223005

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go