OpenOnuAdapter Onu Upgrade command extension to activate fallback and commit active image

Upgrade tests with the Adtran ONU 611 showed possible limitations of the current ONU upgrade API that might make it difficult to enforce the activation/commitment of existing images on the ONU.
Problems may arise from issues within the running image of the ONU concerning the delivery of the ONU upgrade related indications via OMCI. In our case a strange VERSION indication was delivered for the images that were intended to be downloaded. It might also be that the the downloaded image immanent VERSION somehow is not what it is supposed to be.
Current API's only allow activities for the given image VERSION and consistently this is checked throughout the adpater upgrade processing.
In cases like above it can be helpful to enforce the activation of the (fallback) image irrespective to the delivered image VERSION, as we are quite sure that what was downloaded is highly trustworthy. And it might be that without activation of this image the other upgrade procedures may never be successful due to some misbehavior/unexpected behavior in the currently running ONU image.
Accordingly I would suggest the extension of the upgrade API by following commands:
voltctl device onuimage activate-fallback <commitOnSuccess> <device-ids> with return success/error
voltctl -s localhost:50057 device onuimage status-fallback <device-ids> with return ImageState