Allow project roles to be inherited from their parent domain

Role assignments to a domain are already supported, in a similar way
to projects. However, in customer situations where administration of
role assigment is split between a cloud provider and individual domain
administrators, a cloud provider might want to be able to have roles
inherited from the domain (that they create) to the projects (that the
domain administrator creates).

This functionality is being provided as an extension, ahead of a more
integrated solution for IceHouse.

bp inherited-domain-roles

Change-Id: I8471b182337ae85600ac564795a2cc04fc85d1d5