caddytls: Add dns config to acmeissuer

[francislavoie]

The dns option was missing, so this would fail with an error:

    tls {
        dns cloudflare XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
        issuer acme {
            dir https://acme-staging-v02.api.letsencrypt.org/directory
            resolvers 1.0.0.1
        }
    }

Error during parsing: when defining an issuer, all its config must be in its block, rather than from separate tls subdirectives

The fix is to add dns to acmeissuer as well, so this is possible:

    tls {
        issuer acme {
            dir https://acme-staging-v02.api.letsencrypt.org/directory
            dns cloudflare XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
            resolvers 1.0.0.1
        }
    }

[mholt]

Aye, that ought to do it. Can't believe I forgot about that one 😅 Thanks!