Add security context for each incoming request.

Pecan allows hooks for processing requests.
This patch adds a new hook which extracts information from
keystone auth_token, creates a new instance of RequestContext
and adds this context as a part of a request
accesible via request.security_context.

Change-Id: I42539c06b6dc325172e90c32dabc3bd5476a628f
Partly-Implements: blueprint rbac-filtering
Merge "Add security context for each incoming request."