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.
Merge pull request #518 from xliiv/location-form-fields-ok

added location fields to device_info form