Change exception type while deattaching root device

By default it is not possible to deattach volume that is configured
as root device for an instance.
Previous HTTP 403 reponse pointed that caller don't have permission
to deattach the volume instead he's not able to do it. Response 400
suits more for that case.

Change-Id: Ife59900448719f78c0505f1b681cb26674fa0306

nova/api/openstack/compute/volumes.py

@@ -433,7 +433,7 @@ class VolumeAttachmentController(wsgi.Controller):
 
         if bdm.is_root:
             msg = _("Cannot detach a root device volume")
-            raise exc.HTTPForbidden(explanation=msg)
+            raise exc.HTTPBadRequest(explanation=msg)
 
         try:
             self.compute_api.detach_volume(context, instance, volume)

nova/tests/unit/api/openstack/compute/test_volumes.py

@@ -541,7 +541,7 @@ class VolumeAttachTestsV21(test.NoDBTestCase):
                  new_callable=mock.PropertyMock)
     def test_detach_vol_root(self, mock_isroot):
         mock_isroot.return_value = True
-        self.assertRaises(exc.HTTPForbidden,
+        self.assertRaises(exc.HTTPBadRequest,
                           self.attachments.delete,
                           self.req,
                           FAKE_UUID,