Remove logic for CentOS < 9

... because RDO supports CentOS Stream 9 (aka CentOS 9) only since Zed.

This also refactors the logic to determine the Python 3 version and
makes the openstacklib::default class hard-fail in unsupported OS
Families.

Change-Id: I9d1fa698356d7840e50edf9c975e3672be62e912

manifests/defaults.pp

@@ -5,20 +5,15 @@
 # This file is loaded in the params.pp of each class.
 #
 class openstacklib::defaults {
-
-  if ($::os['family'] == 'Debian') {
-    $pyver3 = '3'
-  } elsif $::os['family'] == 'RedHat' {
-    if Integer.new($::os['release']['major']) > 8 {
+  case $::osfamily {
+    'RedHat': {
       $pyver3 = '3.9'
-    } elsif Integer.new($::os['release']['major']) == 8 {
-      $pyver3 = '3.6'
-    } else {
-      $pyver3 = '2.7'
     }
-  } else {
-    # TODO(tkajinam) This is left to keep the previous behavior but we should
-    #                revisit this later.
-    $pyver3 = '2.7'
+    'Debian': {
+      $pyver3 = '3'
+    }
+    default:{
+      fail("Unsupported osfamily: ${::osfamily}")
+    }
   }
 }