Sunbeam control plane only tests fail because no nova-compute

[gboutry]

https://github.com/openstack-charmers/zaza-openstack-tests/blob/891761d631e200dff9ebbc8e97639c54ffc7d6f6/zaza/openstack/charm_tests/tempest/utils.py#L248C5-L248C9

This changed added a dynamic lookup on units nova-compute.

Sunbeam control plane tests deploy no nova-compute application.

Sunbeam deployment have an openstack-hypervisor and not a nova-compute unit.

This change seems tailored toward charmed openstack only

  File "/home/zuul/src/opendev.org/openstack/sunbeam-charms/.tox/func/lib/python3.12/site-packages/zaza/openstack/charm_tests/tempest/utils.py", line 248, in _add_nova_config
    ctxt['min_compute_nodes'] = len(model.get_units('nova-compute'))
                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/zuul/src/opendev.org/openstack/sunbeam-charms/.tox/func/lib/python3.12/site-packages/zaza/__init__.py", line 224, in _wrapper
    return future.result(timeout)
           ^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/concurrent/futures/_base.py", line 456, in result
    return self.__get_result()
           ^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/concurrent/futures/_base.py", line 401, in __get_result
    raise self._exception
  File "/home/zuul/src/opendev.org/openstack/sunbeam-charms/.tox/func/lib/python3.12/site-packages/zaza/__init__.py", line 209, in _runner
    return await f(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/zuul/src/opendev.org/openstack/sunbeam-charms/.tox/func/lib/python3.12/site-packages/zaza/model.py", line 741, in async_get_units
    return model.applications[application_name].units
           ~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
KeyError: 'nova-compute'

[gboutry]

Fixed #1282