Add "max_valid_packages_stored" to worker config (#4116)

* is needed for #4111

Author: jochenehret

lib/cloud_controller/config_schemas/base/worker_schema.rb

@@ -83,6 +83,7 @@ class WorkerSchema < VCAP::Config
 
             packages: {
               max_package_size: Integer,
+              max_valid_packages_stored: Integer,
               app_package_directory_key: String,
               fog_connection: Hash,
               fog_aws_storage_options: Hash,