Let named_vhost listen on URLs automatically.

Now that we support calling apache::listen multiple times on the same
address/port pair, we can let apache::named_vhost automatically listen
on the URLs it will handle.  Do so.

This will make it a little bit easier for users, who will no longer
need to instantiate both an apache::named_vhost resource, *and* an
apache::listen resource.  Perhaps not a huge deal, especially since
we now allow multiple apache::listen resources for the same address/
port pairs, but even a little bit helps.