Clarify API version, auth and password prompts

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Because the password isn't the only interactive bit of information the user may be asked about when adding a remote.

We also have a certificate fingerprint prompt (which we don't have an argument for yet) and various remote backends may ask extra questions (such as system-image asking for the channel prefix).