Fix json encode error during dc host update

`_previous_state` includes non serializable fields, like date/datetime
Reduce data being send only to hostname.
Add also test checking if data is serializable.

src/ralph/data_center/models/physical.py

@@ -356,6 +356,7 @@ class DataCenterAsset(
     Asset
 ):
     _allow_in_dashboard = True
+    previous_dc_host_update_fields = ['hostname']
 
     rack = models.ForeignKey(Rack, null=True, blank=True)
     status = TransitionField(

src/ralph/data_center/publishers.py

@@ -10,7 +10,10 @@ def _get_host_data(instance):
     serializer = DCHostSerializer(instance=instance)
     if hasattr(serializer.instance, '_previous_state'):
         data = deepcopy(serializer.data)
-        data['_previous_state'] = serializer.instance._previous_state
+        data['_previous_state'] = {
+            k: v for k, v in serializer.instance._previous_state.items()
+            if k in serializer.instance.previous_dc_host_update_fields
+        }
     else:
         data = serializer.data
     return data

src/ralph/data_center/tests/test_signals.py

@@ -1,3 +1,5 @@
+import json
+
 from django.test import TestCase
 
 from ralph.data_center.publishers import _get_host_data
@@ -33,3 +35,14 @@ def test_sending_data_includes_previous_data(self):
             data = _get_host_data(getattr(self, obj_name))
             results.append('_previous_state' in data)
         self.assertEqual(results, [True] * 3)
+
+    def test_sending_data_doesnt_raise_json_error(self):
+        for obj_name in ['cloud_host', 'dc_asset', 'virtual_server']:
+            json.dumps(_get_host_data(self.dc_asset))
+
+    def test_sending_data_includes_only_selected_fields(self):
+        data = _get_host_data(self.dc_asset)
+        self.assertEqual(
+            list(data['_previous_state'].keys()),
+            self.dc_asset.previous_dc_host_update_fields,
+        )

src/ralph/virtual/models.py

@@ -153,6 +153,7 @@ def update_service_env_on_cloudproject_save(sender, instance, **kwargs):
 
 
 class CloudHost(PreviousStateMixin, AdminAbsoluteUrlMixin, BaseObject):
+    previous_dc_host_update_fields = ['hostname']
 
     def save(self, *args, **kwargs):
         try:
@@ -289,6 +290,8 @@ class VirtualServer(
     # TODO: remove this field
     cluster = models.ForeignKey(Cluster, blank=True, null=True)
 
+    previous_dc_host_update_fields = ['hostname']
+
     @cached_property
     def polymorphic_parent(self):
         return self.parent.last_descendant if self.parent_id else None