By Elizabeth Piper, Alistair Smout and Andrew MacAskill LONDON (Reuters) -British Prime Minister Boris Johnson was fighting for his political survival on Tuesday after finance minister Rishi Sunak and another senior minister abruptly resigned over the latest scandal to blight his administration. Sunak and health minister Sajid Javid sent resignation letters to Johnson within minutes of each other in which both took aim at his ability to run an administration that adhered to standards. Signalling his intention to stay in power for as long as possible, Johnson quickly appointed former businessma…