After a person copies `hosts.yml` to a headless system, they will still
be forced to go through re-authentication flow unless they copy or
initialize a `config.yml` as well. This fixes respecting authentication
info from `hosts.yml` even if `config.yml` is missing.
Read from and write to the `hosts.yml` file every time `config.yml` is
accessed. Everything that before went under the `hosts:` map now belongs
to `hosts.yml`.
Previously we would trigger OAuth flow when the config file did not
exist. Now we will let an empty Config object be initialized in that
case, but trigger OAuth flow when the Context caller requests an
AuthToken.
