[controller] Return uuid hex instead of str in register()

Searching the str version in the admin doesn't yeld
the expected result, therefore it's better to show
the hex version whenever possible

django_netjsonconfig/controller/generics.py

@@ -145,6 +145,8 @@ def post(self, request, *args, **kwargs):
             'key: {key}\n' \
             'hostname: {name}\n'
         s += 'is-new: %s\n' % (int(new))
-        return ControllerResponse(s.format(**config.__dict__),
+        attributes = config.__dict__
+        attributes['id'] = config.pk.hex
+        return ControllerResponse(s.format(**attributes),
                                   content_type='text/plain',
                                   status=201)