Hostname field

DEPENDS ON: https://github.com/allegro/django-bob/pull/32

Added field *hostname*:
- to model and to forms (DC|BO)(add|edit),
- to bulkedit asset (only readonly),
- to search.

Fixed test test_bulk_edit_max_items_at_once.
Added 'hostname' to api.

@ar4s @quamilek 
Use ``from ralph.ui.widgets import ReadOnlyWidget``