Port the Hosts API's hypervisor-guests mapping endpoint to the Instances API

The Instances API was created and is intended to replace the Hosts API. When this was done, we did not migrate the Host's hypervisor-guest mappings endpoint – GET /hosts/{hypervisor_uuid}/guests

In order to make the Instances API more consistent and to be able to fully deprecate and remove the Hosts API completely, we will need to port the guests portion of the Hosts API to the Instances API.

DONE:

• New guests enpoint added to the Instances API

• • GET /instances/{id}/guests
    • Find and return the mapped guests associated with the specified target host id.
    • id – The id of the swatch hypervisor host record
  • API spec updated and documented
  • Lookup should be based on the internal Host ID as known by swatch.
  • API should reuse InstanceData in the response.