Add list resource provider usage

Add a method to return the current resource usage for a specific
resource provider. Given a specific rp_uuid return the resource
providers current generation as well as its assoicated resource class
usage at that time. Example output below:

(Pdb) self.resource_providers_client.list_resource_provider_usages('90234521-0f4f-4777-98d8-731db8e61a0d')
{'resource_provider_generation': 52, 'usages': {'VGPU': 1}}

More details of the api can be referenced here [1]

[1] https://docs.openstack.org/api-ref/placement/#resource-provider-usages

Change-Id: I13ca77f1cd8fbf74cd716b2d8eae772f5328a4d4
3 files changed