Replace remaining use of legacy facts

... because these are no longer available in Puppet 8.

Change-Id: Ic1f11b7e12d35f7381d621c6f28636a34ca14e0e
(cherry picked from commit ee48d7a366)

spec/acceptance/mysql_spec.rb

@@ -10,7 +10,7 @@ describe 'openstacklib mysql' do
 
       class { 'mysql::server': }
 
-      $charset = $::operatingsystem ? {
+      $charset = $facts['os']['name'] ? {
         'Ubuntu' => 'utf8mb3',
         default  => 'utf8',
       }