Support for storage control units, including storage paths

[andy-maier]

Add support for storage control unit objects, as child objects of the storage subsystem (!) object. The usual methods for creatable objects should be supported.

Note that the API uses the terminology define/undefine instead of create/delete, but I suggest to use the usual create()/delete() methods for consistency.

The standard list() method can be implemented straight forward by using the HMC list operation "list control units of a subsystem".

Add support for the add/remove volume range operations via special methods: add/remove_volume_range().

Add support for the storage path element object as a standard child object of the control unit object via attribute "storage_paths".

The storage path objects should support the standard methods for creatable element objects:

• list() based on the storage-path-uris property
• create()/delete()/update_properties() using the corresponding API operations.
• the usual other methods/attributes.

Add unit, function and end2end testcases, consistent with other objects.

Add the corresponding mock support.

Add mock definitions for some storage control units for some CPCs.

Add testcases for the mock support.