Replace context decorators with context managers

The context decorator needs to have the "context" parameter in the
first position or the first argument must be "self" or "cls",
followed by the "context" object.

Conflicts:
    neutron/objects/db/api.py

Closes-Bug: #2017784
Change-Id: Ib80f7c72e78854226b227e354792320c78fed5d0
(cherry picked from commit 4e27e27)

Author: ralonsoh

neutron/objects/db/api.py

@@ -13,6 +13,7 @@
 # TODO(ihrachys): cover the module with functional tests targeting supported
 # backends
 
+from neutron_lib.db import api as db_api
 from neutron_lib.db import model_query
 from neutron_lib import exceptions as n_exc
 from neutron_lib.objects import utils as obj_utils
@@ -31,16 +32,18 @@ def _get_filter_query(obj_cls, context, query_field=None, query_limit=None,
 
 
 def get_object(obj_cls, context, **kwargs):
-    return _get_filter_query(obj_cls, context, **kwargs).first()
+    with db_api.CONTEXT_READER.using(context):
+        return _get_filter_query(obj_cls, context, **kwargs).first()
 
 
 def count(obj_cls, context, query_field=None, query_limit=None, **kwargs):
-    if not query_field and obj_cls.primary_keys:
-        query_field = obj_cls.primary_keys[0]
-        if query_field in obj_cls.fields_need_translation:
-            query_field = obj_cls.fields_need_translation[query_field]
-    return _get_filter_query(obj_cls, context, query_field=query_field,
-                             query_limit=query_limit, **kwargs).count()
+    with db_api.CONTEXT_READER.using(context):
+        if not query_field and obj_cls.primary_keys:
+            query_field = obj_cls.primary_keys[0]
+            if query_field in obj_cls.fields_need_translation:
+                query_field = obj_cls.fields_need_translation[query_field]
+        return _get_filter_query(obj_cls, context, query_field=query_field,
+                                 query_limit=query_limit, **kwargs).count()
 
 
 def _kwargs_to_filters(**kwargs):