Trusts API (bp trusts)

Credit mostly due to ayoung, based on his work in:

  https://review.openstack.org/#/c/22063/

This patch is simply an evolution of the above.

Trusts introduce delegated authorization and impersonation capabilities
into the v3 API.

Change-Id: I8215cbf39a8aba826419e89d3f10aa354dbf9266