Support alternate 'public-keys' format for NoCloud service.

The 'public-keys' in the metadata is sometimes just a simple list of keys
instead of a list dicts with an 'openssh-key', e.g:

```yaml
instance-id: my-instance-id
local-hostname: my-host-name
public-keys:
  - ssh-rsa ...
```

Change-Id: I46c1228e1e1ea5bbf34144480633907a1ea20317

Author: luqmana

cloudbaseinit/metadata/services/nocloudservice.py

@@ -307,6 +307,9 @@ def get_public_keys(self):
         if not raw_ssh_keys:
             return []
 
+        if isinstance(raw_ssh_keys, list):
+            return raw_ssh_keys
+
         return [raw_ssh_keys[key].get('openssh-key') for key in raw_ssh_keys]
 
     def get_network_details(self):

cloudbaseinit/tests/metadata/services/test_nocloudservice.py

@@ -247,6 +247,16 @@ def test_get_public_keys(self, mock_get_metadata):
         result = self._config_drive.get_public_keys()
         self.assertEqual(result, expected_result)
 
+    @mock.patch(MODULE_PATH + '.NoCloudConfigDriveService._get_meta_data')
+    def test_get_public_keys_alt_fmt(self, mock_get_metadata):
+        fake_key = 'fake key'
+        expected_result = [fake_key]
+        mock_get_metadata.return_value = {
+            'public-keys': [fake_key]
+        }
+        result = self._config_drive.get_public_keys()
+        self.assertEqual(result, expected_result)
+
     @ddt.data(('', ('V2 network metadata is empty', None)),
               ('1', ('V2 network metadata is not a dictionary', None)),
               ('{}', ('V2 network metadata is empty', None)),