named_vhost: Allow overriding auto-listen.

Having the named_vhost definition automatically configure Apache to
listen on the address/port pairs used by the virtual host, is usually
helpful.  There are occasions, however, when it will give unwanted
results, or even break things.  In particular, if you have overlaps,
with one virtual host handling a specific address, and one handling
the "wildcard" address (e.g. https://*/), you can't have Apache listen
to both INADDR_ANY and a specific address.  There may be other cases
where users want more control as well.

We here introduce a new parameter 'listen' on the apache::named_vhost
definition, which specifies addresses (URLs) that Apache httpd should
be configured to listen on.  If not specified, it defaults to using
the same set of addresses as specified by the 'urls' parameter, so in
many cases users won't need to specify it.