Processing LXD results failure with loopback

[Adam Collard]

2021-04-23 10:28:15 metadataserver.api: [critical] autopkgtest.maas(wgsfaa): commissioning script '50-maas-01-commissioning' failed during post-processing.
        Traceback (most recent call last):
          File "/usr/lib/python3/dist-packages/metadataserver/api.py", line 820, in signal
            target_status = process(node, request, status)
          File "/usr/lib/python3/dist-packages/metadataserver/api.py", line 641, in _process_commissioning
            self._store_results(
          File "/usr/lib/python3/dist-packages/metadataserver/api.py", line 529, in _store_results
            script_result.store_result(
          File "/usr/lib/python3/dist-packages/metadataserver/models/scriptresult.py", line 384, in store_result
            signal_status = try_or_log_event(
        --- <exception caught here> ---
          File "/usr/lib/python3/dist-packages/metadataserver/api.py", line 447, in try_or_log_event
            func(*args, **kwargs)
          File "/usr/lib/python3/dist-packages/metadataserver/builtin_scripts/hooks.py", line 1027, in process_lxd_results
            update_node_interfaces(node, data)
          File "/usr/lib/python3/dist-packages/provisioningserver/utils/twisted.py", line 202, in wrapper
            result = func(*args, **kwargs)
          File "/usr/lib/python3/dist-packages/maasserver/utils/orm.py", line 721, in call_within_transaction
            return func_within_txn(*args, **kwargs)
          File "/usr/lib/python3.8/contextlib.py", line 75, in inner
            return func(*args, **kwds)
          File "/usr/lib/python3/dist-packages/metadataserver/builtin_scripts/network.py", line 89, in update_node_interfaces
            settings = interfaces[name]
        builtins.KeyError: 'lo'

This is a regression in master given the recent branches landing

[Adam Collard (adam-collard)]

https://paste.ubuntu.com/p/pG338pnfBB/ contains the commissioning data from this controller