Merged metrics_engine package into decision_engine

In this changeset, I merged the metrics_engine package into
the decision_engine one alongside the required changes to make
the tests pass.

Change-Id: Iac1cd266a854212bf4fa8b21c744b076c3b834a8
Partially-Implements: blueprint cluster-model-objects-wrapper

watcher/tests/decision_engine/test_scheduling.py

@@ -21,8 +21,8 @@ from apscheduler.triggers import interval as interval_trigger
 import eventlet
 import mock
 
+from watcher.decision_engine.loading import default as default_loading
 from watcher.decision_engine import scheduling
-from watcher.metrics_engine.loading import default as default_loading
 from watcher.tests import base
 from watcher.tests.decision_engine.strategy.strategies import \
     faker_cluster_state