monitoring: Use command transports constants in the command transport factory

modules/monitoring/library/Monitoring/Command/Transport/CommandTransport.php

@@ -53,19 +53,21 @@ abstract class CommandTransport
     public static function fromConfig(Zend_Config $config)
     {
         switch (strtolower($config->transport)) {
-            case 'remote':
+            case RemoteCommandFile::TRANSPORT:
                 $transport = new RemoteCommandFile();
                 break;
-            case 'local':
+            case LocalCommandFile::TRANSPORT:
             case '':  // Casting null to string is the empty string
                 $transport = new LocalCommandFile();
                 break;
             default:
                 throw new ConfigurationError(
                     'Can\'t create command transport \'%s\'. Invalid transport defined in \'%s\'.'
-                    . ' Use one of \'local\' or \'remote\'.',
+                    . ' Use one of \'%s\' or \'%s\'.',
                     $config->transport,
-                    self::$config->getConfigFile()
+                    self::$config->getConfigFile(),
+                    LocalCommandFile::TRANSPORT,
+                    RemoteCommandFile::TRANSPORT
                 );
         }
         unset($config->transport);