Supporting cloud service discovery (EC2, Azure etc) for alert managers

[c16a]

The current approach of connecting to alertmanager instances from cortex is to use static IPs, or SRV records. In cases where Amazon Route 53 or other services from other cloud providers are not used, there is no dynamic way to query the provider's API and connect to alert managers.

An example is from Prometheus configuration to fetch nodes to scrape from.