fix: cloud-init datasource_list must be one-line

Ensure datasource_list configuration from subiquity is a single
line: datasource_list: [None] to support ds-identify limitation
in parsing single-line datasource_list values.

cloud-init's ds-identify doesn't actually support multi-line YAML
and never has.

We can see this behavior in /run/cloud-init/ds-identify.log which
opts to use a default datsource_list and logs:

  /etcloud/cloud.cfg.d/99-installer.cfg set datasource_list:
  no datasource_list found, using default: ....

This issue doesn't affect subiquity in practice because
it also places /etc/cloud/ds-identify.cfg containing `policy: enabled`.
Which means cloud-init will be enabled regardless of whether
ds-identify finds a valid datasource.

We see this in /run/cloud-init/ds-identify.log with the following:

  DSNAME=
  ...
  mode=enabled returning 0

Once cloud-init python code runs and merges system configuration in
/etc/cloud/cloud.cfg.d with a proper YAML parser, it correctly parses
multi-line YAML config and successfully limits the merged
datasource_list: [ None ] provided in
/etc/cloud/cloud.cfg.d/99-installer.yaml.

If subiquity were to drop the /etc/cloud/ds-identify.cfg file in the
future, cloud-init would actually be disabled on first boot because it
never detected a viable datasource during systemd generator timeframe
and ds-identify policy doesn't mandate being enabled by default.

subiquity/models/subiquity.py

@@ -435,7 +435,10 @@ class SubiquityModel:
                         "metadata": metadata,
                     },
                 },
-            }
+            },
+            # ds-identify does not support multi-line YAML datasource_list.
+            # Turn off default_flow_style forcing datasource_list: [None].
+            default_flow_style=None,
         )
         grub_dpkg = yaml.dump({"grub_dpkg": {"enabled": False}})
         files = [

subiquity/models/tests/test_subiquity.py

@@ -285,7 +285,8 @@ grub_dpkg:
   enabled: false
 """,
             "etc/cloud/cloud.cfg.d/99-installer.cfg": re.compile(
-                "datasource:\n  None:\n    metadata:\n      instance-id: .*\n    userdata_raw: \"#cloud-config\\\\ngrowpart:\\\\n  mode: \\'off\\'\\\\npreserve_hostname: true\\\\n\\\\\n"  # noqa
+                "datasource:\n  None:\n    metadata: {instance-id: .*}\n    userdata_raw: \"#cloud-config\\\\ngrowpart:\\\\n  mode: \\'off\\'\\\\npreserve_hostname: true\\\\n\\\\\n.*datasource_list: \[None\]",  # noqa
+                re.DOTALL,
             ),
             "etc/hostname": "somehost\n",
             "etc/cloud/ds-identify.cfg": "policy: enabled\n",
@@ -320,7 +321,10 @@ grub_dpkg:
             ):
                 for cpath, content, perms in model._cloud_init_files():
                     if isinstance(expected_files[cpath], re.Pattern):
-                        self.assertIsNotNone(expected_files[cpath].match(content))
+                        self.assertIsNotNone(
+                            expected_files[cpath].match(content),
+                            f"Missing expected file match {cpath}: {content}"
+                        )
                     else:
                         self.assertEqual(expected_files[cpath], content)