OpenOnuAdapter Onu Sw Upgrade internal image storage improvements

Current situation: ONU images stored to the ONU adapter are deleted from local file system only with the according 'onuimage abort' command. But for the operator it may be quite difficult to keep track of which images are still stored to the ONU adapter. There is no system command that provides a way of requesting the actual stored images. So the operator has to keep track of what images where downloaded and should use that infromation to delete the images again at a certain point using the abort command. If this is not done many images could be collected on the ONU adapter wasting unneeded resources.

Following improvements are possible:
a) Each new download command removes the previously existing image. So only one imae is kept in the ONU adapter. This is no preferred solution as it would prevent downloading of different images to devices of different vendors in parallel.
b) The system provides a new API to retrieve the currently stored images in order to use this information to remove the images using the 'onuimage abort' command. This would require modifications in multiple Voltha components (add-on functionality, but also changes in the kind how the image information is stored within the OnuAdapter)
c) The OnuAdapter limits the number of images to one per VendorId. This would at least limit the maximum number of images that can be stored in total. Within the OnuAdapter the file slice could be changed o a map of VendorIDs, which might also have some performance improval within the file access. It requires anyway some re-organisation of the file control code within the OnuAdapter. The change would be possible without touching the other Voltha components.

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