Change instance_uuid to resource_uuid

Use an application-agnostic term for the resource the context is
attached to.

bp/app-agnostic-logging-parameters

Change-Id: I548120770d0f11a2af56f07d0902433a8c7c98fe
Add docstring to get_admin_context()

Document public API function

Change-Id: I445076350ba55a76bc71956b8aa1d14929a83a13
Cache the current context for the thread

Use a threading.local instance to store the current RequestContext, with
an option to not overwrite an existing context.

bp/graduate-oslo-context

Change-Id: I000cb13392ee21258dc2a91683294dc9ff2aeb8f
Add ClearRequestContext fixture

Add a fixture class for use in tests to provide a way to flush the
cached RequestContext without exposing how that is stored.

bp/graduate-oslo-context

Change-Id: I27dd50a61d364e1698823629e2676c10ced50af3
Documentation cleanup

Tidy up filenames, layout, and other minor issues with the documentation
before our first release.

bp/graduate-oslo-context

Change-Id: Ib284bbdb50f868b232abc050311ddd5a9d9bde15