Bugfix for duplicate listen option caused by hardcoded `ipv6only=on` in template

The hardcoded `ipv6only=on` listen option in mailhost.erb, mailhost_ssl.erb, vhost_header.erb and vhost_ssl_header.erb cause nginx to fail:
`duplicate listen options for [::]:80 in /etc/nginx/sites-enabled/ [...]`

To avoid this I moved `ipv6only=on` from the templates into the puppet code. This way it should be backwards compatible if no listen options are passed to the define type.