Make instance_action_events.traceback a non-required field

instance_action_events.traceback can only be seen by admin users
with default policy.json,
"os_compute_api:os-instance-actions:events":
    "rule:admin_api or role:InstanceEvents"
so it should not be deemd as a required field in schema.
https://developer.openstack.org/api-ref/compute/#show-server-action-details

Change-Id: Id85c75347eba2b10b8ecc0ea262fd870281c610d
Closes-Bug: #1783275
diff --git a/tempest/lib/api_schema/response/compute/v2_1/servers.py b/tempest/lib/api_schema/response/compute/v2_1/servers.py
index 2954de0..3979c65 100644
--- a/tempest/lib/api_schema/response/compute/v2_1/servers.py
+++ b/tempest/lib/api_schema/response/compute/v2_1/servers.py
@@ -430,8 +430,9 @@
             'traceback': {'type': ['string', 'null']}
         },
         'additionalProperties': False,
-        'required': ['event', 'start_time', 'finish_time', 'result',
-                     'traceback']
+        # NOTE(zhufl): events.traceback can only be seen by admin users
+        # with default policy.json, so it shouldn't be a required field.
+        'required': ['event', 'start_time', 'finish_time', 'result']
     }
 }