@Override
  public Set<SubResourceDefinition> getSubResourceDefinitions() {
    Set<SubResourceDefinition> setChildren = new HashSet<>();
    return setChildren;
  }
An extension version is like a stack version but it contains custom services.  Linking an extension
version to the current stack version allows the cluster to install the custom services contained in
the extension version.
ambari-server/src/main/java/org/apache/ambari/server/api/resources/ExtensionLinkResourceDefinition.java