Ability to consume keystone events for project delete

Specification Added: https://review.openstack.org/#/c/99658/

Problem:
There is no way to delete barbican tenant for a project delete in Keystone, this will lead to inconsistent data in Barbican.
There has to be someway to sync with keystone delete project event and soft delete the barbican tenant along with related entities.

Use Case:

Delete a project in keystone, barbican tenant corresponding to project has to be deleted in Barbican along with related entities.

Solution:

Keystone event notification framework emits events for such events. Usually these events are published to a topic (or queue per service). Barbican can consume such events and respond to it as needed.

High Level Change

1. Support for keystone event listener.
2. Tenant Repository will be modified to delete with cascade.
3. Something more
4. No extra API needed.