This adds a v2 client class allowing to add support for upcoming
v2 API endpoints. The v2 client class implements all v1 endpoints.
The cloudkitty API version can be specified with the
"--os-rating-api-version" option or the "OS_RATING_API_VERSION"
environment variable.
Change-Id: If38730da3baed59c93543a08f8a4989f919611db