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/tests/conf_fixture.py

@@ -26,9 +26,9 @@ CONF.import_opt('sqlite_synchronous', 'oslo_db.options', group='database')
 
 
 class ConfFixture(fixtures.Fixture):
-    """Fixture to manage global conf settings."""
+    """Fixture to manage conf settings."""
 
-    def __init__(self, conf):
+    def __init__(self, conf=cfg.CONF):
         self.conf = conf
 
     def setUp(self):