oslo.context's initializer should do a better job handing class instantiation

[Amrith Kumar]

oslo.context is designed and intended to be serialized and sent along as a dictionary in a message bus between components that have to be upgraded independently.

When the shape of oslo.context changes, and a project subclasses oslo.context's RequestContext(), the current implementation requires lock-step upgrades of both the sender and receiver.

This is less than optimal and leads to code like Trove's code in context.py to work around this which isn't ideal.