Fix v2 API dataframes get policy check

oslo.policy expects the key project_id, not tenant_id anymore. This was
causing the GET /v2/dataframes policy check to fail:

    $ openstack --rating-api-version 2 rating dataframes get
    {"message": "You are not authorized to perform this action"} (HTTP 403)

In the v1 API, the storage:list_data_frames operation was not affected
because it uses project_id.

Change-Id: Ie4aa6a21e3829223aab0f91d809e311e0f0318cb
Story: 2009879
Task: 44618
(cherry picked from commit 32bf128b81)

cloudkitty/api/v2/dataframes/dataframes.py

@@ -72,7 +72,7 @@ class DataFrameList(base.BaseResource):
         policy.authorize(
             flask.request.context,
             'dataframes:get',
-            {'tenant_id': flask.request.context.project_id},
+            {'project_id': flask.request.context.project_id},
         )
 
         begin = begin or tzutils.get_month_start()

releasenotes/notes/dataframes-get-v2-policy-check-6070fc047b2e1496.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Fixes policy check when getting dataframes using the v2 API, causing the
+    operation to fail when run by a non-admin user. See story 2009879
+    <https://storyboard.openstack.org/#!/story/2009879>`_ for more details.