@ApiModelProperty(name = "extension_name")
    public String getExtensionName() {
      return extensionName;
    }
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/controller/ExtensionLinkResponse.java