Fix logic in test_db_loadbalancer

Several methods in test_db_loadbalancer have incorrect logic like the following piece of code:

res = self._create_vip(fmt,
                                       name,
                                       pool_id,
                                       protocol,
                                       protocol_port,
                                       admin_state_up,
                                       subnet_id=tmp_subnet['subnet']['id'],
                                       **kwargs)
vip = self.deserialize(fmt or self.fmt, res)
if res.status_int >= 400:
       raise webob.exc.HTTPClientError(code=res.status_int)

In case of res.status_int >= 400 code tries to deserialize malformed body and it throws an exception masking real cause of failure.