Trusted Attributes Policy for External Identity Providers

Allows fine grained control over which attributes (both types and values)
Keystone will accept as trustworthy from an external identity provider.
All attributes (types and/or values) that are not trusted will be discarded
by federated Keystone.

implements: bp trusted-attribute-issuing-policy

Co-Authored-By: David Chadwick <d.w.chadwick@kent.ac.uk>

Change-Id: Ib17b19da9fd6a0b88b9d3336d8f663128c8b9cde
Merge "Trusted Attributes Policy for External Identity Providers"