added option default_server to mark one server as the default.

Default server are usefull if you have multiple named server and you want to control which content is delivered if no host is given in the request or an ip address is used for the request.

Eample usage:
  $defaultdir= '/var/lib/nginx/defaultpage'

  file { "${defaultdir}/index.html":
    mode   => '444',
    owner  => 'www-data',
    ensure => 'present',
    content => '<html><head><title>host not found</title></head><body><p>host not found</p></body></html>',
  }

  nginx::resource::vhost { $fqdn:
    ensure   => present,
    default_server  => 'true',
    www_root => $defaultdir,
  }