Implement the Rating Service (CloudKitty) APIs

[mariusleu]

Cloudkitty is a service that facilitates the implementation of a billing system in openstack clouds by rating the usage of resources in a project.
According to the project docs, it has been deployed in several openstack production clusters, therefore it's important that Openstack4j should support it.

API documentation: https://docs.openstack.org/cloudkitty/latest/webapi/v1.html