added empty devices in response
Moved slot validation to view.
Better var name: asset_without_category_and_location.
fixed review sugestions
Merge pull request #521 from ar4s/feature/empty-space-in-api-rack

Added empty devices in response
Merge pull request #522 from xliiv/no-slot-validation-ok2

Moved slot validation to view.
- sync parent also for blase servers
- use slote_no instead position
- update info also for blade servers
Merge pull request #523 from andrzej-jankowski/sync

Location sync - changes
Added Accessory model + AccessoryType choice.
Moved device-info models to seperate file: models_device_info.py
added missing image to doc: doc/images/add-data-center-asset-form.png
Merge pull request #524 from xliiv/feature/accessories-model-ok

Added Accessory Model
Merge pull request #525 from xliiv/added-missing-image

added missing doc image