ZOOKEEPER-4985 Fix zkCleanup.sh script to not fail on non-standard config directories

[lfrancke]

https://issues.apache.org/jira/browse/ZOOKEEPER-4985

The zkCleanup.sh script has several issues that make it difficult to use in containerized or non-standard deployments:

The script hardcodes the path to zoo.cfg and fails with confusing error messages when the config file is not in the default location. When the config file is missing, it passes empty strings to PurgeTxnLog, causing confusing error messages

Steps to Reproduce:

• Run ZooKeeper with config in a non-standard location (e.g., /custom/path/zoo.cfg)
• Try to run cleanup:

grep: /path/to/zookeeper/bin/../conf/zoo.cfg: No such file or directory
grep: /path/to/zookeeper/bin/../conf/zoo.cfg: No such file or directory
Path '/path/to/zookeeper/bin' does not exist.
Usage:
PurgeTxnLog dataLogDir [snapDir] -n count 

[lfrancke]

The shellcheck failures were there before already.