Pass the RequestContext to the handlers

Make sure that one instance of a handler is used for each request
as we are passing in the context in the constructor.

Change-Id: I066090968db733aed93f7b8729e0fec8c83876d0
Use the context to pass to the db

Also make sure the user_id/project_id are assigned from the context
so the user doesn't try and supply a different user/project.

Change-Id: If5caba512db38231a1c7f7c588f5f01a639b609d
Add catalog helper methods to RequestContext

This makes it easier to get at the endpoints within the catalog.
This patch also changes the test to correctly pass in a string
catalog as it is retrieved from the headers.

Change-Id: I5453b356f611ec1dc0341d253cbf29986ae66ce5
Partly-Implements: blueprint rbac-filtering