Add routing key data source

[erikvanbrakel]

Consider the following flow:

Amazon ALB -> Healthy Hosts Metric + Alarm -> SNS -> VictorOps -> Engineers

Alerting from SNS to VictorOps goes through an HTTPS endpoint which is specifically setup to receive SNS notifications. This endpoint will auto-confirm the subscription you configure.

To distinguish between the different flows of alerts / incidents, you can use routing keys. To use them in Terraform, we can pull them out of the API.