DPE-2710 - test: add storage scale down/up storage re-use test

[marcoppenheimer]

Changes Made

fix: actually use storage, persist both transactions + snapshots

• Storage volume was mounted to the charm container, not to the workload. As such, storage wasn't actually persisted
• Resolved by persisting data and data-log dirs under /var/lib/zookeeper
• Although advice is to mount transactions + snapshots on different volumes for IO performance, Juju supports only a single volume. As ZooKeeper uses the Txn log to re-build snapshots after data loss, it's a trade-off for now

test: add scaling storage re-use test

• Checks for existence of 'old' files after a scale-down + up
• If 'old' files present, storage was re-used

[zmraul]

Looks good!