added location fields to device_info form

Details:
Added models to ``admin``:
- data_center
- server_room
- rack

Updated ``clean`` method on ``device_info`` model, so that field constaraints
are respected (see docstring of ``clean``)
Added new form field to dc asset: hostname.
Fixed tests and added new ones.
Maybe "create a Ralph-core device" would be simpler?