I tried to configure the module with a particular SSLCertificateFile ($server_ssl_cert) and SSLCertificateKeyFile ($server_ssl_key) but not configure a SSLCertificateChainFile ($server_ssl_chain) or SSLCACertificateFile ($server_ssl_ca).
As far as I can tell puppet-foreman uses undef for its own defaulting so you cant get Apache's default behavior. So you have to have Apache configured with those two, which seems unfortunate.