DAOS-17305 mgmt: Skip pools during engine start (#16452)

[liw]

Add an "internal" environment variable, DAOS_POOL_BLACKLIST, for skipping problematic pools in emergency situations.

Steps for the author:

• [ ] Commit message follows the guidelines.
• [ ] Appropriate Features or Test-tag pragmas were used.
• [ ] Appropriate Functional Test Stages were run.
• [ ] At least two positive code reviews including at least one code owner from each category referenced in the PR.
• [ ] Testing is complete. If necessary, forced-landing label added and a reason added in a comment.

After all prior steps are complete:

• [ ] Gatekeeper requested (daos-gatekeeper added as a reviewer).

[github-actions[bot]]

Ticket title is 'Skip pools during engine start' Status is 'Resolved' https://daosio.atlassian.net/browse/DAOS-17305

[daosbuild3]

Test stage Functional Hardware Medium completed with status FAILURE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net//job/daos-stack/job/daos/view/change-requests/job/PR-16606/2/execution/node/1434/log