[digitalocean|compute] Added list_flavors mock code

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

@@ -16,7 +16,20 @@ module Fog
       class Mock
 
         def list_flavors
-          Fog::Mock.not_implemented
+          response = Excon::Response.new
+          response.status = 200
+          response.body = {
+            "status" => "OK",
+            "sizes"  => [
+              {"id" => 33,"name" => "512MB"},
+              {"id" => 34,"name" => "1GB"},
+              {"id" => 35,"name" => "2GB"},
+              {"id" => 36,"name" => "4GB"},
+              {"id" => 37,"name" => "8GB"},
+              {"id" => 38,"name" => "16GB"}
+            ]
+          }
+          response
         end
 
       end