Zookeeper: configure 'dataLogDir' in separate disk

http://zookeeper.apache.org/doc/trunk/zookeeperAdmin.html#sc_advancedConfiguration

> Having a dedicated log device has a large impact on throughput and stable latencies. It is highly recommened to dedicate a log device and set dataLogDir to point to a directory on that device, and then make sure to point dataDir to a directory not residing on that device.

http://www.mail-archive.com/user@zookeeper.apache.org/msg07646.html

etcd would get separate wal, data dir as well.

TBD: Consul?

Start with the linked ZooKeeper administration page and mail-archive discussion to understand the separate log and data directory requirement. Then inspect how dbtester handles etcd configuration and determine whether Consul is in scope. Done means the supported databases have clearly defined separate WAL or log and data directories, with the scope documented.

Written by the indexing model from the issue text.