Fix disabled_reason's type in services schema

disable_reason in services schema can be null when status
is enabled, so this is to add null in disable_reason's type.

https://developer.openstack.org/api-ref/compute/#update-compute-service

Change-Id: I5478eff73675d0f236acddfb5387cc8cd9e91a5c
partially-implements: blueprint full-schema-for-all-microversions
diff --git a/tempest/lib/api_schema/response/compute/v2_53/services.py b/tempest/lib/api_schema/response/compute/v2_53/services.py
index aa132a9..97b0c72 100644
--- a/tempest/lib/api_schema/response/compute/v2_53/services.py
+++ b/tempest/lib/api_schema/response/compute/v2_53/services.py
@@ -42,7 +42,8 @@
                 'properties': {
                     'id': {'type': 'string', 'format': 'uuid'},
                     'binary': {'type': 'string'},
-                    'disabled_reason': {'type': 'string'},
+                    # disabled_reason can be null when status is enabled.
+                    'disabled_reason': {'type': ['string', 'null']},
                     'host': {'type': 'string'},
                     'state': {'type': 'string'},
                     'status': {'type': 'string'},