[digitalocean|compute] Added list_images mock code

lib/fog/digitalocean/requests/compute/list_images.rb

@@ -16,7 +16,31 @@ module Fog
       class Mock
 
         def list_images
-          Fog::Mock.not_implemented
+          response = Excon::Response.new
+          response.status = 200
+          response.body = {
+            "status" => "OK",
+            "images" => [
+              # Sample image
+              {
+                "id" => 1601, 
+                "name" => "CentOS 5.8 x64", 
+                "distribution" => "CentOS"
+              },
+              {
+                "id" => 1602, 
+                "name" => "CentOS 5.8 x32", 
+                "distribution" => "CentOS"
+              },
+              {
+                "id" => 2676,
+                "name" => "Ubuntu 12.04 x64 Server", 
+                "distribution" => "Ubuntu"
+              },
+
+            ]
+          }
+          response
         end
 
       end