protected List<StageWrapper> beforeBuild(UpgradeContext upgradeContext) {
    List<StageWrapper> stageWrappers = new ArrayList<>(100);
    return stageWrappers;
  }
Performs any pre-processing that needs to be performed on the list of stage
wrappers.

@param upgradeContext
         the upgrade context (not {@code null}).
@return the initial list of stage wrappers, or an empty list (never
        {@code null}).
ambari-server/src/main/java/org/apache/ambari/server/stack/upgrade/orchestrate/StageWrapperBuilder.java