Selection of "default" certificate for non-SNI clients

[mithrandi]

Currently, we use txsni, which allows for a DEFAULT.pem, but txacme will try to issue a certificate for DEFAULT when it tries to renew this, which will fail. We should have a better way for specifying which certificate to use for this.

Possibly related to #37 as both of these will require slightly more information than we are fitting into hostnames currently.

[mithrandi]

One workaround at the moment is to make DEFAULT.pem be a symlink to actualhostname.com.pem; this will result in an error when the certificate is expiring (~every two months), but this can just be ignored.