Split DataGenerator class for v2 and v3

Currently we are working on splitting identity V2 and V3 clients
into the corresponding service clients like users_client, roles_client,
projects_client, etc. In order to do it more smoothly and simpler it was
decided to split DataGenerator class in tempest/api/identity/base.py
into the corresponding classes for Keystone V2 and V3.
So the DataGenerator class was split into three classes:
1. BaseDataGenerator. This class contains common variables and methods
   for Keystone V2 and V3 API.
2. DataGeneratorV2. This class contains specific methods for Keystone
   V2 API.
3. DataGeneratorV3. This class contains specific methods for Keystone
   V3 API.

Change-Id: I60a8d59b418d0ec24250911f449c7ed0e14ae737
5 files changed