make ES doc_type customizable

As for ES "id" parameters, "doc_type" should be customizable,
there is no reason why es document type should match cw_etype

cubicweb_elasticsearch/entities.py

@@ -97,6 +97,10 @@
     def es_id(self):
         return self.entity.eid
 
+    @property
+    def es_doc_type(self):
+        return self.entity.cw_etype
+
     @cachedproperty
     def fulltext_indexable_attributes(self):
         eschema = self._cw.vreg.schema[self.entity.cw_etype]

cubicweb_elasticsearch/hooks.py

@@ -99,7 +99,7 @@
             # IFullTextIndex serializer, therefore the "id" and "doc_type" in
             # kwargs below must be container data.
             kwargs['id'] = serializer.es_id
-            kwargs['doc_type'] = getattr(serializer, 'doc_type', json['cw_etype'])
+            kwargs['doc_type'] = serializer.es_doc_type
             try:
                 # TODO option for async ?
                 es.index(**kwargs)