Add more methods for collecting cluster hostnames

Right now we can configure cluster hostnames via:

- `HAProxySettings`
- `StaticHostsSettings`

Two additional low hanging fruit settings can be:

- via URL: expect plaintext listing of hostnames given a specific URL
- via [orchestrator](https://github.com/github/orchestrator): provide URI for orchestrator such as `https://my.orchestrator.com/api/cluster/alias/mycluster` -- `freno` will parse the JSON response correctly.
- also via `cluster-pool-instances/:clusterName/:pool` (should also support _alias_)