API Microversioning

This patch set adds API microversion support along
with the first API microversion: start/end time for
CONTINUOUS audits.

APIImpact

Implements: blueprint api-microversioning
Depends-On: I6bb838d777b2c7aa799a70485980e5dc87838456
Change-Id: I17309d80b637f02bc5e6d33294472e02add88f86

api-ref/source/samples/audit-show-response.json

@@ -48,5 +48,7 @@
     "strategy_name": "workload_stabilization",
     "next_run_time": "2018-04-06T11:56:00",
     "updated_at": "2018-04-06T11:54:01.266447+00:00",
-    "hostname": "controller"
-}
+    "hostname": "controller",
+    "start_time": null,
+    "end_time": null
+}