By Kate Holton, Elizabeth Piper and Muvija M LONDON (Reuters) – Scandal-ridden Boris Johnson announced on Thursday he would quit as British prime minister after he was abandoned by ministers and most of his Conservative lawmakers. Bowing to the inevitable as more than 50 ministers quit and lawmakers said he must go, an isolated and powerless Johnson spoke outside his Downing Street to confirm he would resign. “The process of choosing that new leader should begin now. And today I have appointed a cabinet to serve, as I will until a new leader is in place,” Johnson said. After days of battling f…