Inherit Context 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.

This patch uses the Context class from oslo-incubator (which will be
soon replaced by oslo.context) as the base class for Sahara Context.
The goal is to be ready for the first oslo.context release.

Change-Id: If7abc705b431aa97cd9d1c59f863d15e6a4fbdef
Partially implements: blueprint app-agnostic-logging-parameters
Merge "Inherit Context from oslo"