Add a --conduit-token parameter to arc
Summary: Ref T5955. This makes it easier to write scripts which call Conduit via arc call-conduit.
Test Plan: Used arc --conduit-token ... call-conduit user.whoami to make calls as various users.
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: mbishopim3, epriestley
Maniphest Tasks: T5955
Differential Revision: https://secure.phabricator.com/D12717