location fields in form

In general: added location fields to device_info model.

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.

@ar4s @quamilek