The OpenStack identity sevice issues domain-scoped tokens and
keystonemiddlware sets a specific header when it validates a
domain-scoped token. This commit allows context objects to set
domain_id attributes for tokens scoped to a domain and also processes
the domain ID header from keystonemiddlware.
Change-Id: I620d647499abaf5a7103d82af22a190fcc0b3fae