Fix doc for service list

If a service is enabled, the "disabled_reason" field in response
for /v2/.{tenant_id}./os-services/detail is null, however, currently
it's specified as "" in the doc, thus may be confusing to users.

This patch change the template and doc, it should not cause issue to
existing applications.

Change-Id: Ia71ec4c97a355bcc1a7b63e6107db77f80a5d843
Close-bug: 1328382
Merge "Fix doc for service list"