Use $ubuntu_url as default value of $backports_url on Ubuntu

Ubuntu shouldn't be using debian backports by default.

This was written by Anoine Beaupré, but split from the commit "move
backports to snippets" since the change is unrelated and needs to be
more visible in the commit history.

manifests/params.pp

@@ -6,13 +6,16 @@ class apt::params () {
   $use_next_release = false
   $debian_url = 'http://httpredir.debian.org/debian/'
   $security_url = 'http://security.debian.org/'
+  $ubuntu_url = 'http://archive.ubuntu.com/ubuntu'
   $backports_url = $::debian_codename ? {
     'squeeze'  => 'http://backports.debian.org/debian-backports/',
-    default => $debian_url
+    default => $::operatingsystem ? {
+      'Ubuntu' => $ubuntu_url,
+      default  => $debian_url,
+    }
   }
   $lts_url = $debian_url
   $volatile_url = 'http://volatile.debian.org/debian-volatile/'
-  $ubuntu_url = 'http://archive.ubuntu.com/ubuntu'
   $repos = 'auto'
   $custom_preferences = ''
   $disable_update = false