Add support for certbot 2.x

[elvisangelaccio]

See https://github.com/certbot/certbot/wiki/Certbot-v2.x-Plugin-Compatibility

[jacqueminv]

For anyone landing here when trying to use certbot-s3front and facing the following error (as reported on #55): unrecognized arguments: --certbot-s3front:auth-s3-bucket, here is one way work around it waiting this issue to be closed:

> virtualenv venv
> source venv/bin/activate
> pip install certbot===1.32.0
> pip install acme===1.32.0
> pip install certbot-s3front

# then as mentioned on the README.md
>  AWS_ACCESS_KEY_ID="REPLACE_WITH_YOUR_KEY" \
AWS_SECRET_ACCESS_KEY="REPLACE_WITH_YOUR_SECRET" \
certbot --agree-tos -a certbot-s3front:auth \
--certbot-s3front:auth-s3-bucket REPLACE_WITH_YOUR_BUCKET_NAME \
[ --certbot-s3front:auth-s3-region your-bucket-region-name ] #(the default is us-east-1, unless you want to set it to something else, you can delete this line) \
[ --certbot-s3front:auth-s3-directory your-bucket-directory ] # (default is "") \
-i certbot-s3front:installer \
--certbot-s3front:installer-cf-distribution-id REPLACE_WITH_YOUR_CF_DISTRIBUTION_ID \
-d REPLACE_WITH_YOUR_DOMAIN