Add more methods for collecting cluster hostnames

[shlomi-noach]

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: 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)