More modern module configuration.

Users have previously been required to supply a class named
x509certs::siteconfig, containing a variable $hostcert_source.
We are replacing that with a parametrized class x509certs::config
which users are instead required to instantiate, providing suitable
parameter values.  This is more inline with modern Puppet practice,
and users no longer need to define a class belonging to this module's
namespace.

THIS IS NOT BACKWARDS-COMPATIBLE!

Users must update their manifests when upgrading, removing their old
x509certs::siteconfig class, and instantiating the x509certs::config
class instead.