Update Server Consolidation global efficacy

As instance migration cost is petty compared to the cost of
compute node release, I update the way to compute the global
efficacy for a server consolidation goal. The new formula is simplest
and it's only based on compute node.

Change-Id: Ibcce31a85af70429f412c96c584a761d681366a2

watcher/decision_engine/goal/efficacy/indicators.py

@@ -104,6 +104,20 @@ class MigrationEfficacy(IndicatorSpecification):
             voluptuous.Range(min=0, max=100), required=True)
 
 
+class ComputeNodesCount(IndicatorSpecification):
+    def __init__(self):
+        super(ComputeNodesCount, self).__init__(
+            name="compute_nodes_count",
+            description=_("The total number of enabled compute nodes."),
+            unit=None,
+        )
+
+    @property
+    def schema(self):
+        return voluptuous.Schema(
+            voluptuous.Range(min=0), required=True)
+
+
 class ReleasedComputeNodesCount(IndicatorSpecification):
     def __init__(self):
         super(ReleasedComputeNodesCount, self).__init__(

watcher/decision_engine/goal/efficacy/specs.py

@@ -33,20 +33,21 @@ class ServerConsolidation(base.EfficacySpecification):
 
     def get_indicators_specifications(self):
         return [
+            indicators.ComputeNodesCount(),
             indicators.ReleasedComputeNodesCount(),
             indicators.InstanceMigrationsCount(),
         ]
 
     def get_global_efficacy_indicator(self, indicators_map=None):
         value = 0
-        if indicators_map and indicators_map.instance_migrations_count > 0:
+        if indicators_map and indicators_map.compute_nodes_count > 0:
             value = (float(indicators_map.released_compute_nodes_count) /
-                     float(indicators_map.instance_migrations_count)) * 100
+                     float(indicators_map.compute_nodes_count)) * 100
 
         return efficacy.Indicator(
             name="released_nodes_ratio",
             description=_("Ratio of released compute nodes divided by the "
-                          "number of VM migrations."),
+                          "total number of enabled compute nodes."),
             unit='%',
             value=value,
         )