Port the Datastore and DatastorePath objects

This patch ports the Datastore and DatastorePath objects which have
been approved in Nova and will be useful in Glance.
Finally, it adds the units module from oslo-incubator.

Change-Id: Ia519a83ed09fde74be376981f89284af20aa1dab
Merge "Port the Datastore and DatastorePath objects"