Add RBAC test for force-deleting a backup

This patch adds a new test for force-deleting a backup in
test_volumes_backup_rbac, which tests the cinder policy action:
"volume_extension:backup_admin_actions:force_delete".

This patch also corrects the currently wrong policy action for
the test test_snapshot_force_delete from
"volume_extension:volume_admin_actions:force_delete" to
"volume_extension:snapshot_admin_actions:force_delete". (The
``_force_delete`` function in the cinder admin actions API enforces
the policy action corresponding to the resource name [0][1]).

[0] https://github.com/openstack/cinder/blob/master/cinder/api/contrib/admin_actions.py#L133
[1] https://github.com/openstack/cinder/blob/master/cinder/api/contrib/admin_actions.py#L82

Change-Id: Icecf2152fe84da2847ae677a74a4d183eb4cd88b
diff --git a/releasenotes/notes/add-force-delete-backup-test-7e896affd1471328.yaml b/releasenotes/notes/add-force-delete-backup-test-7e896affd1471328.yaml
new file mode 100644
index 0000000..3d81baa
--- /dev/null
+++ b/releasenotes/notes/add-force-delete-backup-test-7e896affd1471328.yaml
@@ -0,0 +1,11 @@
+---
+features:
+  - |
+    Added an RBAC test for force-deleting a backup which enforces the cinder
+    policy action: "volume_extension:backup_admin_actions:force_delete".
+fixes:
+  - |
+    Corrected the policy action in the ``rbac_rule_validation`` decorator
+    for the test ``test_snapshot_force_delete`` from
+    "volume_extension:volume_admin_actions:force_delete" to
+    "volume_extension:snapshot_admin_actions:force_delete".