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/cmd/decisionengine.py

@@ -22,11 +22,11 @@ import sys
 
 from oslo_config import cfg
 from oslo_log import log as logging
-from oslo_service import service
 
 from watcher._i18n import _LI
 from watcher.common import service as watcher_service
 from watcher.decision_engine import manager
+from watcher.decision_engine import scheduling
 from watcher.decision_engine import sync
 
 LOG = logging.getLogger(__name__)
@@ -43,5 +43,10 @@ def main():
     syncer.sync()
 
     de_service = watcher_service.Service(manager.DecisionEngineManager)
-    launcher = service.launch(CONF, de_service)
+    bg_schedulder_service = scheduling.DecisionEngineSchedulingService()
+
+    # Only 1 process
+    launcher = watcher_service.launch(CONF, de_service)
+    launcher.launch_service(bg_schedulder_service)
+
     launcher.wait()