Inverted condition for IPv6 warning  

If $ipv6_enable is true ypu sshould warn the user only if the machine _doesn't have_ an ip address.

At the moment the warning is raised when ipv6 is configured properly
Fix ipv6 support

- set $ipv6_listen_options default to undef
- correctly detect the presence of ipaddress6 fact in vhost_header.erb template
- check $ipv6_enable as a boolean in vhost_header.erb