Allow wal and data to be split onto different devices.

[sfitts]

The installation instructions for influx recommend putting the wal and data directories on different devices (https://docs.influxdata.com/influxdb/v1.7/introduction/installation/#hardware-requirements-for-influxdb). However, looking at this chart that's not currently possible as the configuration settings share the same root directory and there is only the provision for a single PVC. It would be nice if the chart offered an option for doing a more scalable deployment.