variable exitcode is not been cleaned before every domain validation

[rafael747]

https://github.com/hamburml/docker-flow-letsencrypt/blob/68747ea5ab4255da7bd93d3e7e554ce2efd56393/certbot.sh#L41-L46

This way, after a initial failure of domain validation, the exitcode variable are only redefined when the validation fails again. If the validation succeeds, the exitcode remains with the value of previous iteration because the "break" statement.

This cause the domain to not be included in the certificate generation.

To work correctly, the exitcode variable needs to be cleaned before every domain validation trial