Add low level api

* api.api.BaseAPI holds the common operations
* api.compute.APIv2, api.identity_v2APIv2, api.identity_v3.APIv3 and
  api.object_store.APIv1 represent the versioned REST API interfaces

* Changes to OSC include adding the API object to the existing client objects,
  although in time these could replace the client objects.

* The existing Object-Store commands in OSC have been moved to openstackclient.api
* A couple of Compute commands have been added to demonstrate the CRUD operations
* An Identity command was implemented for both v2 and v3 to work out versioning

* Tests for the API-layer are included

Change-Id: If0bf70adc18ceb5782237fc7f4f8d8cf1936a4c7