Don't require email and password in hubic backend except for first time

[Pablo Castellano]

When you configure Duplicity using Hubic backend you are required to create ~/.hubic_credentials with login data (email and password) and api data (client_id, client_secret, redirect_uri.
More info here: http://duplicity.nongnu.org/duplicity.1.html#sect16

The first time you run duplicity, it will generate a ~/.hubic_tokens file with two new values named access_token and refresh_token.

Since now you email and password are not used anymore but they are still present in the configuration file. This is insecure because a malicious user could read this file and compromise your whole hubic account.

Proposed workaround:
Once duplicity has obtained the tokens, set email and password to blank or random data