Support ISO boot for partitions

[andy-maier]

The partitions have a 'boot-iso-image-name' property but that is not writeable. Instead, the "Mount ISO image" and "Unmount ISO image" operations are used to change it.

As a result, the zhmc_partition module today has no support for booting the partition from an ISO image.

This could for example be done by artificially making the 'boot_iso_image_name' paramater of the zhmc_partition module writeable and to perform the respective operations inside of the module.

Note, there is also issue #495 that could be addressed once that is supported.

[andy-maier]

This was opened in this repo by mistake. Issue zhmcclient/zhmc-ansible-modules#551 has been opened in the right repo for this.