Add hostname to API Reference

This patch set adds hostname attribute to Audit and
ActionPlan resources in API Reference.

Change-Id: I187d44dc401d3d1770452022b8610570d459cc98

api-ref/source/samples/actionplan-start-response.json

@@ -18,5 +18,6 @@
     "uuid": "4cbc4ede-0d25-481b-b86e-998dbbd4f8bf",
     "audit_uuid": "7d100b05-0a86-491f-98a7-f93da19b272a",
     "created_at": "2018-04-10T11:59:12.592729+00:00",
-    "deleted_at": null
+    "deleted_at": null,
+    "hostname": null
 }