Initialize $release, $next_release, $codename, $next_codename in init.pp.

manifests/init.pp

@@ -16,6 +16,21 @@ class apt {
     require => undef,
   }
 
+  # init $release, $next_release, $codename, $next_codename
+  case $lsbdistcodename {
+      '': {
+          include lsb
+          $codename = $lsbdistcodename
+          $release = $lsbdistrelease
+      }
+      default: {
+          $codename = $lsbdistcodename
+          $release = debian_release($codename)
+      }
+  }
+  $next_codename = debian_nextcodename($codename)
+  $next_release = debian_nextrelease($release)
+
   case $custom_sources_list {
     '': {
       include apt::default_sources_list