Remove default auth client, make config

Don't have a default authentication client. Rationale: If user wants an authentication client, they would have to configure an authentication client (set user/password on it) and then set it onto the stream config. (Also to be more consistent with java stream client).

Also make ```Config#read_from_file``` allow a file that doesn't have all fields (optional fields).

This works with: https://github.com/caskdata/cdap-clients/pull/50.