Update Repository entity

Add the new added checksum_type field

Closes #1571
@omaciel The API layer is currently incapable of sending `null` to the server. That can only be done if requests (or our "client" wrapper) is used directly. See #1082, especially [this comment](https://github.com/SatelliteQE/robottelo/issues/1082#issuecomment-49925850).