Differentiate between Keystone V2 and V3 session data

This allows openstack request classes to know the difference between
Keystone v2 and v3 auth information (aka session data). This logic is
kept within a base request class that all other OpenStack request
classes inherit from. No further changes are required from inheriting
request classes.

Change-Id: I881cb45516814928249825c4dbee77bc5a5840f7