Fix failing copy_image flow init

The new networkx requirement apparently causes us to fail the
test_init_copy_flow_as_non_owner test because a None value can not be
added to the flow graph. This just mocks out the glance_store call
that is failing to get the staging store to avoid us choking there
so we can proceed. We are just testing get_flow so we never use it
anyway.

Closes-Bug: #1959361

Change-Id: I6fcb14ad240105ed0b1b9086c6c6c95034b4dd42
(cherry picked from commit ded8ecf)

Author: kk7ds

glance/tests/unit/async_/flows/test_api_image_import.py

@@ -549,7 +549,8 @@ def setUp(self):
                                               overwrite=False)
 
     @mock.patch("glance.async_.flows.api_image_import.image_import")
-    def test_init_copy_flow_as_non_owner(self, mock_import):
+    @mock.patch('glance_store.get_store_from_store_identifier')
+    def test_init_copy_flow_as_non_owner(self, mock_gs, mock_import):
         img_repo = mock.MagicMock()
         admin_repo = mock.MagicMock()