[ISSUE #2280] Disk ratio return -1.0 when cluster deployed on DLedger mode

[maixiaohai]

What is the purpose of the change

issue #2280

Brief changelog

Fix commit log path not exists

Verifying this change

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[coveralls]

Coverage increased (+0.02%) to 51.32% when pulling 3583df8 on maixiaohai:dledger/fix_disk_ratio_return_wrong into 9f95a97 on apache:master.