Make auth_keys to list keys from all users

Changed ssh.auth_keys to list keys from all users when user is not
defined. Change is backward compatible. When user is provided the
function returns keys from the user only (however not inside user as
user_keys).

Motivation was to use this in mining. I thought that it is a good idea to make auth_keys work in similar way as the new user_keys. However this changes existing functionality and I understand if this raises a discussion.