choose the best SSL cert from among the installed certificates; use the server certificate instead of self-signed certificates

For HTTPS for the non-primary domains, instead of selecting an SSL certificate by expecting it to be in a directory named after the domain name (with special-case lookups
for www domains, and reusing the server certificate where possible), now scan all of the certificates that have been installed and just pick the best to use for each domain.

If no certificate is available, don't create a self-signed certificate anymore. This wasn't ever really necessary. Instead just use the server certificate.