[api] Fixes for swagger

I had to introduce these changes to fix exceptions raised by swagger.

openwisp_network_topology/api/views.py

@@ -4,9 +4,14 @@
     BaseNetworkGraphView,
     BaseReceiveTopologyView,
 )
+from django_netjsongraph.api.serializers import NetworkGraphSerializer
 
 from ..models import Snapshot, Topology
 
+# TODO: use swappable model loading to define this at serializer level
+NetworkGraphSerializer.Meta.model = Topology
+NetworkGraphSerializer.Meta.fields = '__all__'
+
 
 class NetworkCollectionView(BaseNetworkCollectionView):
     queryset = Topology.objects.filter(published=True)