[AWS] Enable authenticating proxy on draft-origin
[AWS] Enable authenticating proxy on draft-origin

- This was set to the wrong node class (`s_cache`, not `s_draft_cache`), so the
  unauthenticated redirect to Signon wasn't working (and causing Smokey to fail).

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet