By William James and Elizabeth Piper LONDON (Reuters) -Boris Johnson and his former finance minister Rishi Sunak were leading potential contenders to replace British Prime Minister Liz Truss on Friday, with candidates canvassing support to become Conservative Party leader in a fast-tracked contest. Truss, chosen by party members to succeed Johnson after he was ousted by his lawmakers in July, quit herself on Thursday after six turbulent weeks in power. Those who want to replace her must secure 100 nominations from Conservative lawmakers by Monday to run in a contest which the party hopes will …