Add low-level API base class

Adds the foundation of a low-level REST API client.  This is the final prep
stage in the conversion of the object-store commands from the old restapi
interface to the keystoneclient.session-based API.

* api.api.BaseAPI holds the common operations

Change-Id: I8fba980e3eb2d787344f766507a9d0dae49dcadf