Add auto_trigger support to watcher

This patch set adds support of auto-triggering of action plans.

Change-Id: I36b7dff8eab5f6ebb18f6f4e752cf4b263456293
Partially-Implements: blueprint automatic-triggering-audit

watcher/common/exception.py

@@ -258,6 +258,12 @@ class ActionPlanReferenced(Invalid):
                 "multiple actions")
 
 
+class ActionPlanIsOngoing(Conflict):
+    msg_fmt = _("Action Plan %(action_plan)s is currently running. "
+                "New Action Plan %(new_action_plan)s will be set as "
+                "SUPERSEDED")
+
+
 class ActionNotFound(ResourceNotFound):
     msg_fmt = _("Action %(action)s could not be found")