Revert "Remove cell unhealthy alert"
Revert "Remove cell unhealthy alert"

- This alert is for periodic healthcheck for the rep
- This is different from BOSH's notion of unhealthiness

This reverts commit d20e205d6b9fec495a0919c6644ec758034c66b7.

pivotal-cf-experimental/datadog-config-oss,pivotal-cf-experimental/datadog-config-oss