apollo-core/src/main/java/com/ctrip/framework/foundation/internals/provider/DefaultServerProvider.java/DefaultServerProvider/getCustomizedServerPropertiesPath
class DefaultServerProvider:  private String getCustomizedServerPropertiesPath() {
    // 1. Get from System Property
    final String serverPropertiesPathFromSystemProperty = System
        .getProperty("apollo.path.server.properties");
    if (!Strings.isNullOrEmpty(serverPropertiesPathFromSystemProperty)) {
      return serverPropertiesPathFromSystemProperty;
    }

    // 2. Get from OS environment variable
    final String serverPropertiesPathFromEnvironment = System
        .getenv("APOLLO_PATH_SERVER_PROPERTIES");
    if (!Strings.isNullOrEmpty(serverPropertiesPathFromEnvironment)) {
      return serverPropertiesPathFromEnvironment;
    }

    // last, return null if there is no custom value
    return null;
  }