Set default en_US.UTF-8 (voxpupuli#671, voxpupuli#689)

This sets a (somewhat US-centric) default of en_US.UTF-8 for LC_ALL in
$environment_variables, to work around a warning given via Elixir in
newer version of the RabbitMQ CLI tools.

manifests/params.pp

@@ -135,7 +135,7 @@
   $ldap_config_variables               = {}
   $wipe_db_on_cookie_change            = false
   $cluster_partition_handling          = 'ignore'
-  $environment_variables               = { 'LC_ALL' => 'C' }
+  $environment_variables               = { 'LC_ALL' => 'en_US.UTF-8' }
   $config_variables                    = {}
   $config_kernel_variables             = {}
   $config_management_variables         = {}