[controller] Device name changed by a template is not updated in the device model instance

We have a template to change the device hostname based on the location where it's in use:

{
    "system": [
        {
            "config_name": "system",
            "config_value": "system",
            "hostname": "{{ LOCATION_NAME }}"
        }
    ]
}

There's one issue with this, though:

The device name in the device model instance is not updated because:

• it is not sent to the controller by openwisp-config, for example by update_info: https://github.com/openwisp/openwisp-config/blob/master/openwisp-config/files/openwisp.agent#L353
• it's not included in UPDATABLE_FIELDS: https://github.com/openwisp/openwisp-controller/blob/64598a775d641d706da4ac22cf5ffbd7a3889792/openwisp_controller/config/controller/views.py#L129

I think it's a valid usecase to update the device hostname with a template and thus have this change reflected in the device model instance.

@nemesisdesign What do you think?

Read openwisp-config/files/openwisp.agent around update_info and openwisp_controller/config/controller/views.py around UPDATABLE_FIELDS. Trace how the templated hostname reaches the controller and device model instance. Done means the model instance reflects the hostname supplied by the template.