LCOW: Refactor to multiple layer-stores based on feedback

Signed-off-by: John Howard <jhoward@microsoft.com>

daemon/graphdriver/windows/windows.go

@@ -931,8 +931,6 @@ func parseStorageOpt(storageOpt map[string]string) (*storageOptions, error) {
 				return nil, err
 			}
 			options.size = uint64(size)
-		default:
-			return nil, fmt.Errorf("Unknown storage option: %s", key)
 		}
 	}
 	return &options, nil