When cluster.instance is defined, use it to namespace S3 objects

Summary: Ref T7163. This isn't technically necessary but seems generally desirable.

Test Plan: Will deploy S3 in production.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7163

Differential Revision: https://secure.phabricator.com/D11770