modify add_cluster.go and add new examples

[fukiki]

Description of the Change

• Correct a syntax error in add_cluster.go

• Add new examples: add_fence_agent.go add_iscsi_data_storage_domain.go add_openstack_image_provider.go add_role.go add_vm_pool.go assign_permission.go assign_permission_to_vms.go asynchronous_inventory.go change_host_cluster.go data_center_maintenance.go list_tags.go deactivate_disk_attachment.go disable_compression.go download_vm_ovf.go export_template.go export_template_as_ova.go export_vm_as_ova.go extend_disk.go get_display_ticket.go get_vm_ip.go import_vm_from_ova.go import_vm_from_vmware.go page_vms.go poll_events.go print_vm_mac_duplicates.go remove_numa_nodes_from_vm.go set_vm_serial_number.go show_summary.go sparsify_disk.go start_vm_with_boot_devices.go test_connection.go upgrade_host.go vm_backup.go

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[imjoey]

Hi @huihui0311 , could you please squash the commits? Thank you.

[imjoey]

@huihui0311 Thank you so much for your great contributions. Your work is definitely a tremendous amount of work. Let's merge them, thank again. 😄