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
Move object-store commands to low-level API

api.object_store.APIv1 now contains the formerly top-level functions
implementing the object-store REST client. This replaces the old-style
ObjectClientv1 that is no longer necessary.

Change-Id: I7d8fea326b214481e7d6b24119bd41777c6aa968