Remove usage of vif_id_on_device for Interface model queries

- this would ensure a user would always get back the Interface.id instead of Interface.vif_id_on_device sometimes
    - this was done by removing vif_id_on_device from Interface.virtual_interface_id method
- updated ipam_model and ipam_service tests
- exposed ability to set model id property for testing purposes
    - if not provided, id is set to UUID as before
- still able to update Interface.vif_id_on_device via update method for storage in melange database