Add named_vhost TLS configuration.

To make it easier for people to configure named virtual hosts doing
HTTPS, add a parameter 'certname' to apache::named_vhost, specifying
names of the certificate files to use for that virtual host.  When
given, TLS will be enabled for the vhost, and configuration directives
pointing out the certificate files (cert, chain and private key) will
be added to the virtual host definition.

Users need to install the certificate files themselves by means of
the x509certs::hostcert::separate from the x509certs Puppet module
from NSC, though.