public StackId getTargetStack() {
    RepositoryVersionEntity repo = m_targetRepositoryMap.values().iterator().next();
    return repo.getStackId();
  }
Gets the single target stack for the upgrade.  By definition, ALL the targets,
despite the versions, should have the same stack.  The source stacks may be different
from the target, but all the source stacks must also be the same.
<p/>

@return the target stack for this upgrade (never {@code null}).
ambari-server/src/main/java/org/apache/ambari/server/stack/upgrade/orchestrate/UpgradeContext.java