Prometheus Alert-Generator Compliance testing for Cortex

[jeromeinsf]

As per Prometheus dev mailing list we need to add Cortex config template test suite so Cortex is automatically tested for compatibility over Prometheus APIs.

Thanks @codesome for the ping

[stale[bot]]

This issue has been automatically marked as stale because it has not had any activity in the past 60 days. It will be closed in 15 days if no further activity occurs. Thank you for your contributions.