Adapt workload_stabilization strategy to new datasource backend

This patch set:
1. Removes nova, ceilometer and gnocchi properties.
2. Adds using of datasource_backend properties along with
   statistic_aggregation method.
3. Changes type of datasource config.

Change-Id: I4a2f05772248fddd97a41e27be4094eb59ee0bdb
Partially-Implements: blueprint watcher-multi-datasource

watcher/tests/decision_engine/model/gnocchi_metrics.py

@@ -21,6 +21,13 @@ class FakeGnocchiMetrics(object):
     def empty_one_metric(self, emptytype):
         self.emptytype = emptytype
 
+    # TODO(alexchadin): This method is added as temporary solution until
+    # all strategies use datasource_backend property.
+    def temp_mock_get_statistics(self, resource_id, metric, period, aggregate,
+                                 granularity=300):
+        return self.mock_get_statistics(resource_id, metric, granularity,
+                                        0, 0, aggregation='mean')
+
     def mock_get_statistics(self, resource_id, metric, granularity,
                             start_time, stop_time, aggregation='mean'):
         result = 0