feature(BlueGreen): Allow preview service to point ONLY at preview ReplicaSet

[marcinjahn]

Summary

Currently, the preview service point either at preview ReplicaSet (if we're in the middle of a rollout), or at the active ReplicaSet (any other time).
I think it'd be useful to have an option to specify that the preview service would never point at the active ReplicaSet. Therefore, if there's no rollout going on, the preview service would not connect you to any pods. It could return something like HTTP 503 instead.

Use Cases

• running tests on preview environments - it'd be a good feedback to know that there is no preview service being deployed at the moment
• it'd be convenient to have confidence that the response I'm seeing for an API request to a preview service is actually coming from the preview service. Currently, I can never be sure.

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