Fix workload_stabilization unavailable nodes and instances

This patch set excludes nodes and instances from auditing
if appropriate metrics aren't available.

Change-Id: I87c6c249e3962f45d082f92d7e6e0be04e101799
Closes-Bug: #1736982

watcher/tests/decision_engine/model/gnocchi_metrics.py

@@ -175,6 +175,8 @@ class FakeGnocchiMetrics(object):
 
         # node 3
         mock['Node_6_hostname_6'] = 8
+        # This node doesn't send metrics
+        mock['LOST_NODE_hostname_7'] = None
         mock['Node_19_hostname_19'] = 10
         # node 4
         mock['INSTANCE_7_hostname_7'] = 4
@@ -188,7 +190,10 @@ class FakeGnocchiMetrics(object):
         if uuid not in mock.keys():
             mock[uuid] = 8
 
-        return float(mock[str(uuid)])
+        if mock[str(uuid)] is not None:
+            return float(mock[str(uuid)])
+        else:
+            return mock[str(uuid)]
 
     @staticmethod
     def get_average_usage_instance_cpu(*args, **kwargs):
@@ -215,6 +220,8 @@ class FakeGnocchiMetrics(object):
 
         # node 4
         mock['INSTANCE_7'] = 4
+
+        mock['LOST_INSTANCE'] = None
         if uuid not in mock.keys():
             mock[uuid] = 8