Specific exception for stale cluster state was added.

Specific exception should be thrown when cluster state
is stale. Current usage is to raise this exception if
compute_model.state is True.
Bug was describeid by Jean-Emile DARTOIS.

Change-Id: Iaddb4cc8007c51bb14759c9da829751e834499d0
Closes-Bug: #1621855

watcher/decision_engine/strategy/strategies/base.py

@@ -185,6 +185,9 @@ class BaseStrategy(loadable.Loadable):
         if not self._compute_model:
             raise exception.ClusterStateNotDefined()
 
+        if self._compute_model.stale:
+            raise exception.ClusterStateStale()
+
         return self._compute_model
 
     @classmethod