[digitalocean|compute] added list_servers mock code

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

@@ -16,7 +16,24 @@ module Fog
       class Mock
 
         def list_servers
-          Fog::Mock.not_implemented
+          response = Excon::Response.new
+          response.status = 200
+          response.body = {
+            "status" => "OK",
+            "droplets"  => [
+              # Sample droplet
+              #{ 
+              #  "backups_active" => nil,
+              #  "id"             => 100823,
+              #  "image_id"       => 420,
+              #  "name"           => "test222",
+              #  "region_id"      => 1,
+              #  "size_id"        => 33,
+              #  "status"         => "active"
+              #}
+            ]
+          }
+          response
         end
 
       end