Datanode volume DU reserved percent should have a non-zero default value.

Currently there are two ways to reserve space in datanode volumes:

1. hdds.datanode.dir.du.reserved.percent allows specifying a percentage of the volume's space to be unused. It applies to all volumes
2. hdds.datanode.dir.du.reserved allows specifying a map of volume name to bytes reserved. Since it depends on a volume path, it cannot have a default value.

By default Ozone should not allow datanode volumes to get 100% full. This can cause the drive to "lock up" because some operations like block delete that would free up space still need extra disk space before they can complete because they must append to the RocksDB WAL.

Add a default value for hdds.datanode.dir.du.reserved.percent to prevent this from happening. Ideally we could reserve a fixed size regardless of drive size, but we would need to re-wrok the configs before we can do that.