sunbeam cluster list not implemented for MAAS deployments

[Nobuto Murata]

Following the Sunbeam MAAS tutorial:
https://microstack.run/docs/multi-node-maas

The `sunbeam cluster deploy` step didn't complete (some units didn't get ready). After that I cannot see the statue with `sunbeam cluster list`.

$ sunbeam --verbose cluster list

...

           DEBUG Plugin validation: Command validation registered in group enable base.py:449
           DEBUG Plugin validation: Command validation registered in group disable base.py:449
           DEBUG Registering plugin images-sync base.py:411
           DEBUG Validating commands: <bound method EnableDisablePlugin.commands of <sunbeam.plugins.images_sync.plugin.ImagesSyncPlugin object at 0x796dd056a680>> base.py:287
           DEBUG Validation successful base.py:311
           DEBUG Registered groups: {'cluster': <CatchGroup cluster>, 'configure': <Group configure>, 'deployment': <CatchGroup deployment>, 'deployment.add': base.py:417
                    <CatchGroup add>, 'deployment.machine': <CatchGroup machine>, 'deployment.network': <CatchGroup network>, 'deployment.space': <CatchGroup space>,
                    'deployment.zone': <CatchGroup zone>, 'disable': <CatchGroup disable>, 'disable.tls': <Group disable-tls>, 'enable': <CatchGroup enable>,
                    'enable.tls': <Group enable-tls>, 'inspect': <Group inspect>, 'manifest': <CatchGroup manifest>, 'proxy': <CatchGroup proxy>, 'repo': <CatchGroup
                    repo>, 'utils': <CatchGroup utils>, 'init': <CatchGroup init>}
           DEBUG Plugin images-sync: Command images-sync registered in group enable base.py:449
           DEBUG Plugin images-sync: Command images-sync registered in group disable base.py:449
           DEBUG [get] https://192.168.151.101:7000/1.0/config/Plugin-repo, args={'allow_redirects': True} service.py:120
           DEBUG https://192.168.151.101:7000 "GET /1.0/config/Plugin-repo HTTP/1.1" 404 125 connectionpool.py:474
           DEBUG Response(<Response [404]>) = {"type":"error","status":"","status_code":0,"operation":"","error_code":404,"error":"ConfigItem not service.py:122
                    found","metadata":null}

           DEBUG ConfigItem not found plugin.py:165
           DEBUG Registering external repo plugins [] plugin.py:365
           DEBUG utils.py:283
                    Traceback (most recent call last):
                      File "/snap/openstack/507/lib/python3.10/site-packages/sunbeam/utils.py", line 277, in __call__
                        return self.main(*args, **kwargs)
                      File "/snap/openstack/507/lib/python3.10/site-packages/click/core.py", line 1078, in main
                        rv = self.invoke(ctx)
                      File "/snap/openstack/507/lib/python3.10/site-packages/click/core.py", line 1688, in invoke
                        return _process_result(sub_ctx.command.invoke(sub_ctx))
                      File "/snap/openstack/507/lib/python3.10/site-packages/click/core.py", line 1688, in invoke
                        return _process_result(sub_ctx.command.invoke(sub_ctx))
                      File "/snap/openstack/507/lib/python3.10/site-packages/click/core.py", line 1434, in invoke
                        return ctx.invoke(self.callback, **ctx.params)
                      File "/snap/openstack/507/lib/python3.10/site-packages/click/core.py", line 783, in invoke
                        return __callback(*args, **kwargs)
                      File "/snap/openstack/507/lib/python3.10/site-packages/sunbeam/provider/maas/commands.py", line 749, in list_nodes
                        raise NotImplementedError
                    NotImplementedError
           WARNING An unexpected error has occurred. Please run 'sunbeam inspect' to generate an inspection report. utils.py:288
           ERROR Error: