DAOS-18618 tests: reduce io size

[wangshilong]

Skip-func-hw-test-medium-ucx-provider: false
Test-tag: test_daos_rebuild_simple
Test-repeat: 5

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 'daos_test/suite.py:DaosCoreTest.test_daos_rebuild_simple - test timeout w/ DER_CSUM err'
Status is 'In Progress'
Labels: 'ci_2.6_daily,ci_master_daily,pr_test,test_2.8'
https://daosio.atlassian.net/browse/DAOS-18618

[daosbuild3]

Test stage Functional Hardware Medium UCX Provider completed with status FAILURE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net//job/daos-stack/job/daos/view/change-requests/job/PR-17771/1/execution/node/1433/log