Add a service registration feature to the Containers resource

Allow services to register as "interested parties" on Barbican Containers. This information would then be available when the container is retrieved, which a client process could use to either warn about these interested parties prior to performing a delete operation, or else use to determine what parties to update if a new Container is created that need to replace an old one. The LBaaS team also wishes that Barbican *prevent* deletions of Containers if one or more interested parties is registered.