Accept a config file path instead of a config name

Config name is a Viper concept used for searching a specific file
in various paths with various extensions.

Making it configurable is usually not a useful feature
as users mostly want to define a full or relative path
to a config file.

This change replaces config name with config file.

cmd/portable.go

@@ -124,7 +124,6 @@ Please take a look at the usage below to customize the serving parameters`,
 			}
 			service := service.Service{
 				ConfigDir:     filepath.Clean(defaultConfigDir),
-				ConfigFile:    defaultConfigName,
 				LogFilePath:   portableLogFile,
 				LogMaxSize:    defaultLogMaxSize,
 				LogMaxBackups: defaultLogMaxBackup,