conduit: suuport oauth tokens in addition to certs

Summary: We're migrating towards the new tech.

Test Plan: tried deleting 'cert' from my .arcrc, still works

Reviewers: medson, #mercurial

Reviewed By: medson

Subscribers: medson, mjpieters

Differential Revision: https://phabricator.intern.facebook.com/D6376729

Signature: 6376729:1511302323:24066afd17b14c13100b70df9c0fca9220e71799