private synchronized static AmbariManagementController getController() {
    if (controller == null) {
      controller = AmbariServer.getController();
    }
    return controller;
  }
Returns a map where the keys are the host group names and the values the
collection of FQDNs the hosts in the host group
@param hostGroups
@return
ambari-server/src/main/java/org/apache/ambari/server/topology/LogicalRequest.java