modules_munin_host.pp

class munin::host {
  Munin::Node_config <<||>> {
    target => $confdir,
  }
}

modules_munin_node_config.pp

define munin::node_config($target = "/etc/munin/munin-conf.d", $group = "default", $address = $name) {
  file { "${target}/${name}.conf" :
    ensure => present,
    owner => "root",
    group => "root",
    content => template("munin/node_config.conf.erb"),
  }
}

nodes.pp

node xyz {
  @@munin::node_config { $fqdn :
    group => $munin_group,
    address => extlookup('external-hostname', $fqdn),
  }
}