trust creation allowed with empty roles list

The docs state "A project_id may not be specified without at least
one role, and vice versa.", however /OS-TRUST/trusts does allow you
to create a trust with an empty roles list and project_id specified.

This results in 401 responses whenever you try to consume the trust,
because there are no roles for the trustee on the authorized project.

This patch will add a check in trust creation to ensure at least one
role exists if project_id is supplied.

Change-Id: Iebad0b6b7ed62a029d1e50afb003679bafb1655d
Closes-Bug: #1214064
Merge "trust creation allowed with empty roles list"