Add support for customizing the repos list (main, contrib, etc.)

The templates already made use of it, but the code didn't set a default value.

README

@@ -107,6 +107,12 @@ These variables allow to override the default APT mirrors respectively
 used for the standard Debian archives, the Debian security archive,
 the Debian official backports and the Debian Volatile archive.
 
+$apt_repos
+----------
+
+If this variable is set the default repositories list ("main contrib non-free")
+is overriden.
+
 Classes
 =======
 

manifests/init.pp

@@ -42,6 +42,10 @@ class apt {
     ''      => 'http://volatile.debian.org/debian-volatile/',
     default => "${apt_volatile_url}",
   }
+  $repos = $apt_repos ? {
+    ''      => 'main contrib non-free',
+    default => "${apt_repos}",
+  }
 
   package { apt:
     ensure => installed,