Add the user- prefix to the users username for all external requests

Any time the GUI needs to talk to juju-core it needs the 'user-' prefix to the username which we don't require the user to input when logging in. 
This condition sounds confusing. I think that either we always store the user with the prefix or we never do it. (FWIW I'd prefer the former). So credentials.user.indexOf('user-') !== 0 should always return the same result, i.e. should not be required.