private Collection<String> getAdditionalServices(Map<String, String> requestInfoProperties) {
    if (requestInfoProperties != null) {
      String value = requestInfoProperties.get(DIRECTIVE_ADDITIONAL_SERVICES);

      if(!StringUtils.isEmpty(value)) {
        return Arrays.asList(value.split("\\s*,\\s*"));
      }
    }

    return null;
  }
Get the optional list of services indicated as <code>additional_servides</code> which are to be
used in addtion to currently installed services when evaluating <code>when</code> clauses.

@param requestInfoProperties a map a request info properties
@return a collection of service names, or <code>null</code> if not specified
ambari-server/src/main/java/org/apache/ambari/server/controller/internal/ClusterKerberosDescriptorResourceProvider.java