* each service class was moved to a new file
* Service and VersionedService were moved from api_discovery to service_base.py
* api_discovery.py is removed and methods for discovery were moved to a newly
created class Services - class holds methods related to instantiating
services, discovering their versions and extensions, configuring them
* constants were moved to an independent file - constants.py
Change-Id: I00880f4bd30cc4d1609c20aecca820854312b1e7
