cbuildbot has "health_alert_recipients" and "health_threshold" configuration values.

Perhaps we can slightly extend luci-notify to generate similar alerts, and let us delete relevant logic from cbuildbot.

The problem will be that luci-notify can only be configured to alert on a per LUCI builder name, while the cbuildbot builds care about the build config name instead. The two values are unrelated, though the build config is available as the buildbucket property "cbb_config".