Add framework for generating keystone trust

Magnum needs framework to generate keystone trust, which will be injected into bay nodes (which are nova instances). Bay node will use keystone trusts to authenticate itself to an OpenStack service (Swift, Barbican, Neutron, etc.). The reason of using keystone trust (not username/password) is that the scope of trust can be limited in some extent (e.g. scope to a specific service, assign a specific role, etc.).

There are potential three use cases of keystone trust:
1. TLS support [1]: A kubernetes bay might need to store/retrieve secrets from barbican.
2. Docker registry v2 support [2]: A bay need to store docker image into swift.
3. External load balancer [3]: A kubernetes bay needs to talk to neutron for external load balancing.

[1] https://review.openstack.org/#/c/194905/
[2] https://blueprints.launchpad.net/magnum/+spec/registryv2-in-master
[3] https://blueprints.launchpad.net/magnum/+spec/external-lb