New location fields in dc asset 

Added location fields to device_info model.
Renamed old device_info.rack to rack_old.

@andrzej-jankowski @ar4s 
Asset could be in:
* data center
* server's room (server room is in data center)
* rack (rack is in some server's room)

I think that this model must have foreign keys to: data_center, server_root, rack