DAOS-18930 test: remove hardcoded ib0 from mem_ratio

[daltonbohning]

Remove hardcoded ib0/ib1 from mem_ratio.yaml.

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]

Ticket title is 'src/test/ftest/pool/mem_ratio should not hardcode ib0'
Status is 'Open'
https://daosio.atlassian.net/browse/DAOS-18930