private Map<String, String> getSystemStreamProperties(String systemName, String streamName) {
    if (systemName == null) {
      Collections.emptyMap();
    }
    SystemConfig systemConfig = new SystemConfig(this);
    Config defaults = systemConfig.getDefaultStreamProperties(systemName);
    Config explicitConfigs = subset(String.format(STREAM_PREFIX, systemName, streamName), true);
    return new MapConfig(defaults, explicitConfigs);
  }
Finds the properties from the legacy config style (config key includes system).
This will return a Config with the properties that match the following formats (if a property is specified through
multiple formats, priority is top to bottom):
1) "systems.{systemName}.streams.{streamName}.{property}"
2) "systems.{systemName}.default.stream.{property}"

@param systemName the system name under which the properties are configured
@param streamName the stream name
@return           the map of properties for the stream
samza-core/src/main/java/org/apache/samza/config/StreamConfig.java