Added DE Background Scheduler w/ model sync jobs

In this changeset, I implemented a background scheduler service
for Watcher and more particularly for the Decision Engine where
I made it create 2 types of job per cluster data model collector
plugin:

- An initial job that is asynchronously executed upon starting the
  Decision Engine
- A periodical job that gets triggered every configurable interval
  of time

Change-Id: I3f5442f81933a19565217b894bd86c186e339762
Partially-Implements: bluprint cluster-model-objects-wrapper

watcher/common/exception.py

@@ -324,6 +324,10 @@ class MetricCollectorNotDefined(WatcherException):
     msg_fmt = _("The metrics resource collector is not defined")
 
 
+class ClusterDataModelCollectionError(WatcherException):
+    msg_fmt = _("The cluster data model '%(cdm)s' could not be built")
+
+
 class ClusterStateNotDefined(WatcherException):
     msg_fmt = _("The cluster state is not defined")