Don't listen on port or set NameVirtualHost for non-existent vhost

Apache should not listen on ports or define NameVirtualHost directives
for which there is no virtual host defined. Discussed in isse #465.
Merge pull request #470 from antaflos/no_listen_namevirtualhost_without_vhost

Don't listen on port or set NameVirtualHost for non-existent vhost