Allow Standalone Trusts without Trustee User

Currently, Keystone trusts require a trustee user to delegate the trust to. This way they are only usable in a scenario where they are delegated to a service user or to a dedicated user created by a service user. This adds a lot of user administration overhead with little security benefit. This blueprint adds a way to tie a trust to a standalone API key that can be used to acquire a token scoped to the trust without requiring a user account.