Inherit RequestContext from oslo

In the long-term, OpenStack projects should use oslo.context as the base
class for their custom contexts. This enables the use of
application-agnostic tools, e.g. global logging configurations. See
http://specs.openstack.org/openstack/oslo-specs/specs/juno/app-agnostic-logging-parameters.html

This patch syncs the latest context module from oslo-incubator and use
it as the base class for Cinder RequestContext. The goal is to clean up
Cinder code, and to be ready for the first oslo.context release.

Change-Id: Iba9354b402de556ca6e55e7c17dd6f3f1624d72d
Related-bug: #1210261
Merge "Inherit RequestContext from oslo"