add context to keystonemiddleware

this pulls in context module from oslo as it's required for
notifications (which will be used in the audit middleware).

syncs context file to: Id66aba0c600896521a4a236e0a97859735419357

Implements: blueprint audit-middleware
Change-Id: Id983775129dcc795ab91d37d3ba7d90cc6f6a7a9
Merge "add context to keystonemiddleware"