Fixes #5802 - HC update/create using system uuids

Alter Host Collection api update/create actions to accept system uuids.
This is a fix related to hammer-cli-katello commands where some of
the commands take uuids and others take the data base id. This allows
the cli commands to uniformly take uuids.

* also added class_name to relation for system in SystemHostCollection
Merge pull request #4175 from dustint-rh/host_collections_create_take_uuid

Fixes #5802 - HC update/create using system uuids