abstract the apt preferences so that custom preferences can be set,

while maintaining a sane default preferences set can be set. This is
done in the same manner that the custom/default sources_list are handled

manifests/init.pp

@@ -41,6 +41,19 @@ class apt {
 		}
 	}
 
+        case $custom_preferences {
+          '': {
+            include default_preferences
+          }
+          default: {
+            config_file { "/etc/apt/preferences":
+              content => $custom_preferences
+              alias => apt_config,
+              require => File["/etc/apt/sources.list"];
+            }
+          }
+        }
+        class default_preferences {
 	  config_file {
 	    # this just pins unstable and testing to very low values
 	    "/etc/apt/preferences":
@@ -55,6 +68,7 @@ class apt {
 	      content => "APT::Get::Show-Upgraded true;\nDSelect::Clean $real_apt_clean;\n",
 	      before => File[apt_config];
 	  }
+        }
 
 	$apt_base_dir = "/var/lib/puppet/modules/apt"
 	modules_dir { apt: }