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/workload_balance.py

@@ -283,6 +283,9 @@ class WorkloadBalance(base.WorkloadStabilizationBaseStrategy):
         if not self.compute_model:
             raise wexc.ClusterStateNotDefined()
 
+        if self.compute_model.stale:
+            raise wexc.ClusterStateStale()
+
         LOG.debug(self.compute_model.to_string())
 
     def do_execute(self):