By Elizabeth Piper, Kate Holton and Alistair Smout LONDON (Reuters) – Cabinet ministers gathered in Boris Johnson’s official residence on Wednesday to tell him to quit as British leader, after he insisted he would not stand down in the face of a mounting rebellion within his party. With more than 30 government resignations and many lawmakers in his Conservative Party in open revolt, some senior ministers were in Downing Street to tell the prime minister he needed to go, a source said. At least one was there to back him if he decided to fight on, another source said. Despite the clamour for him…