Support simpler, token-based Conduit authentication in Arcanist

Summary:
Ref T5955. If the server supports token-based authentication, prefer it over certificate-based authentication.

Also fixes T3117.

Test Plan:
  - Used `arc install-certificate` to install credentials from both token-based and certificate-based hosts.
  - Used `arc list` with a token-based host.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T3117, T2878, T5955

Differential Revision: https://secure.phabricator.com/D10988
Merge branch 'master' of github.com:facebook/arcanist