Add capabilities property to the DataDriver

This patch adds the capabilities property to DataDrivers as a first step
to support storage capabilities. For now, capabilities are static for
storage drivers, whereas they are calculated dynamically for pooled
drivers.

Follow up patches will make drivers capabilities dynamic.

Change-Id: I29602ab5c8b3857caa3a5a6b1e9e37a5f88b2eef
Partial-blueprint: expose-storage-capabilities