Register acme-dns account even when renewal isn't due

[emilazy]

Currently, an acme-dns account for a domain is only registered when renewal is due. This means that migrating an existing certificate to acme-dns only registers an account at the last minute when renewal is due, which is an unfortunate user experience since it requires manual intervention to set up the _acme-challenge CNAME records.

I think it would be better to unconditionally register when lego (run|renew) is run and the account data isn't present, so that you can migrate certificates to acme-dns in bulk.