LONDON (Reuters) -Britain on Tuesday said it had temporarily relaxed rules around airport slots to allow airlines to devise realistic flight schedules and avoid last minute cancellations in light of staff shortages. A spate of flight cancellations has caused widespread disruption for holidaymakers and served as an ominous omen for the summer, with transport minister Grant Shapps telling airlines to stop selling flights they cannot deliver. But the industry and politicians have pointed out that the government had demanded that airlines put on full flight schedules, previously saying that airlin…