node junglist {
  $override_site = "override"
  $site_id = "junglist"

  if $override_site {
    $sitename = $override_site
  } else {
    $sitename = "sgk_${site_id}"
  }

  notify { $sitename: }
}

aj@junglist /tmp$ ./test.pp --debug
debug: Creating default schedules
debug: //Node[junglist]/Notify[override]: Changing message
debug: //Node[junglist]/Notify[override]: 1 change(s)
notice: override
debug: Finishing transaction -610221608 with 1 changes

node junglist {
#  $override_site = "override"
  $site_id = "junglist"

  if $override_site {
    $sitename = $override_site
  } else {
    $sitename = "sgk_${site_id}"
  }

  notify { $sitename: }
}

aj@junglist /tmp$ ./test.pp --debug
debug: Creating default schedules
debug: //Node[junglist]/Notify[sgk_junglist]: Changing message
debug: //Node[junglist]/Notify[sgk_junglist]: 1 change(s)
notice: sgk_junglist
debug: Finishing transaction -609984798 with 1 changes