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
1 file changed