Fix api doc format
Fix api doc format

- The 'Template List' is a secondary title.
- The headers in the request parameters of the Webhook Add are
 json string, default empty should be "",  not null.

Change-Id: I4c7a22cceff8d95121e55d294cfbcb635e2c730d

openstack/vitrage,openstack/vitrage,openstack/vitrage