private List<String> gatherHosts(ClusterTopology clusterTopology) {
    List<String> hosts = Lists.newArrayList();
    for (Map.Entry<String, HostGroupInfo> entry : clusterTopology.getHostGroupInfo().entrySet()) {
      hosts.addAll(entry.getValue().getHostNames());
    }
    return hosts;
  }
Recommend configurations by the stack advisor, then store the results in cluster topology.
@param clusterTopology cluster topology instance
@param userProvidedConfigurations User configurations of cluster provided in Blueprint + Cluster template
ambari-server/src/main/java/org/apache/ambari/server/api/services/stackadvisor/StackAdvisorBlueprintProcessor.java