This patch adds two new vendor_passthru methods for Redfish:
- create_subscription (create a sbuscription)
- delete_subscription (delete a subscription)
- get_all_subscriptions (get all subscriptions on the node)
- get_subscription (get a single subscription)
Unit Tests in test_utils split into multiple classes to avoid random
failures due to cache.
Tested in bifrost env using two different HW:
- HPE EL8000 e910
- Dell R640
Story: #2009061
Task: #42854
Change-Id: I5b7fa99b0ee64ccdc0f62d9686df655082db3665
4bc5142df2