[maintenance_manager-test] small clean-up log retention scenario
[maintenance_manager-test] small clean-up log retention scenario

Updated the expected string to be resistant to memory usage
fluctuations.  The motivation for this change is seeing sporadic
failures due to the discrepancy in reported memory usage: e.g.,
2.00% instead of 0.00% was reported by dist-test here:
  http://dist-test.cloudera.org/job?job_id=aserbin.1591087472.56061

Change-Id: I4c0a80412f6e6b0d8d594e3310e6d6e26c12267a
Reviewed-on: http://gerrit.cloudera.org:8080/16025
Tested-by: Kudu Jenkins
Reviewed-by: Andrew Wong <b68e4fdc6430321a6b47400732ff97d7ae91234e@cloudera.com>

helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu