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/decision_engine/strategy/strategies/workload_balance.py

@@ -265,7 +265,7 @@ class WorkloadBalance(base.WorkloadStabilizationBaseStrategy):
         """
         LOG.info(_LI("Initializing Workload Balance Strategy"))
 
-        if self.compute_model is None:
+        if not self.compute_model:
             raise wexc.ClusterStateNotDefined()
 
     def do_execute(self):