Cleanup RequestContext
Cleanup RequestContext

Now oslo_context.context has a lot of useful method [1],
So we should avoid to reinvent the wheel.

[1]: http://docs.openstack.org/developer/oslo.context/api/context.html

Change-Id: I6409b6784a126fa44c3b4e660afda1c57175d341

kevin-zhaoshuai/zun,kevin-zhaoshuai/zun,kevin-zhaoshuai/zun