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/base.py

@@ -40,6 +40,7 @@ import abc
 import six
 
 from watcher.common import clients
+from watcher.common import exception
 from watcher.common.loader import loadable
 from watcher.common import utils
 from watcher.decision_engine.loading import default as loading
@@ -176,6 +177,9 @@ class BaseStrategy(loadable.Loadable):
                 'compute', osc=self.osc)
             self._compute_model = collector.get_latest_cluster_data_model()
 
+        if not self._compute_model:
+            raise exception.ClusterStateNotDefined()
+
         return self._compute_model
 
     @classmethod