Add playbook.arguments to the /playbooks list view

Making playbook arguments available in the list view is a compromise
which increases the size of the response but prevents a call to the
playbook detailed view if the only purpose was to retrieve the playbook
arguments.

For example, this would allow ara-web to provide more context in the
playbook list view (ex: if --check was enabled) without needing additional
API calls for each playbook.

Change-Id: I32c2abfb2770b240ac2ea98c14ce054b922a4dfc

ara/api/serializers.py

@@ -281,8 +281,9 @@ class ListLabelSerializer(serializers.ModelSerializer):
 class ListPlaybookSerializer(DurationSerializer, ItemCountSerializer):
     class Meta:
         model = models.Playbook
-        exclude = ("arguments", "created", "updated")
+        exclude = ("created", "updated")
 
+    arguments = ara_fields.CompressedObjectField(default=ara_fields.EMPTY_DICT, read_only=True)
     labels = SimpleLabelSerializer(many=True, read_only=True, default=[])