Add strategy template doc

Developer should provide a detailled documentation about his strategy
algorithm to make it even easier to use by a Watcher end user.
I propose in this changeset a template for strategy documentation.
you will find also a example with basic consolidation strategy.

Change-Id: I66da1a33b87a94b508dd23ac7dce4cae6f4e068b

watcher/decision_engine/goal/efficacy/specs.py

@@ -37,9 +37,9 @@ class ServerConsolidation(base.EfficacySpecification):
             indicators.InstanceMigrationsCount(),
         ]
 
-    def get_global_efficacy_indicator(self, indicators_map):
+    def get_global_efficacy_indicator(self, indicators_map=None):
         value = 0
-        if indicators_map.instance_migrations_count > 0:
+        if indicators_map and indicators_map.instance_migrations_count > 0:
             value = (float(indicators_map.released_compute_nodes_count) /
                      float(indicators_map.instance_migrations_count)) * 100