Create assets from existing devices

[matejv]

On existing installations when user first installs inventory plugin it would be useful to quickly populate assets from existing devices, modules and inventory items and then establish relation to correct asset.

I'm thinking this might be best implemented as a django management command.

We could add a few options to limit what to import as asset: only for certain status values, only if serial present... A dry-run option is mandatory.

For inventory items this will be a bit more challenging as we need to create InventoryItem Types as well. I'm thinking that creating one for each (inventoryitem.manufacturer, inventoryitem.part_id) combo might do.