Service Layer: Vendor Extensions

Vendor extensions are not an immediate priority work item. They should be excluded from the core library and reside in a separate namespace, TBD. Polymorphism is achieved in two ways: allowing vendors to override service clients with their own concrete classes (i.e. HpComputeClient) or through injecting a new json-schema definition into the base client, offering a different API.

Along with vendor extensions, each provider should also plan to have a set of tests that cover functionality only pertinent to their implementation of OpenStack.