Add examples for Hub Manager/Agent to alluxio-env.sh
Add examples for Hub Manager/Agent to alluxio-env.sh

### What changes are proposed in this pull request?

Follow-up to https://github.com/Alluxio/alluxio/pull/14991. Adding env
examples for Hub Manager/Agent.

### Why are the changes needed?

Adding missing examples for Hub manager and agent.

### Does this PR introduce any user facing changes?

No

pr-link: Alluxio/alluxio#15013
change-id: cid-1dc434a08ca09424e9055eb240e2a0f083e4cea3
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