Renamed api.py to base.py in metrics engine

In watcher/metrics_engine/cluster_history/api.py,
we can find the BaseClusterHistory abstract base class.

To follow the same naming convention observed throughout the rest
of the project, I renamed watcher/metrics_engine/cluster_history/api.py
to watcher/metrics_engine/cluster_history/base.py

Change-Id: If18f8db7f0982e47c1998a469c54952670c262f5
Closes-Bug: #1548398

watcher/metrics_engine/cluster_history/ceilometer.py

@@ -22,13 +22,13 @@ from oslo_config import cfg
 from oslo_log import log
 from watcher.common import ceilometer_helper
 
-from watcher.metrics_engine.cluster_history import api
+from watcher.metrics_engine.cluster_history import base
 
 CONF = cfg.CONF
 LOG = log.getLogger(__name__)
 
 
-class CeilometerClusterHistory(api.BaseClusterHistory):
+class CeilometerClusterHistory(base.BaseClusterHistory):
     def __init__(self, osc=None):
         """:param osc: an OpenStackClients instance"""
         super(CeilometerClusterHistory, self).__init__()

watcher/metrics_engine/cluster_model_collector/nova.py

@@ -23,12 +23,12 @@ from watcher.decision_engine.model import hypervisor as obj_hypervisor
 from watcher.decision_engine.model import model_root
 from watcher.decision_engine.model import resource
 from watcher.decision_engine.model import vm as obj_vm
-from watcher.metrics_engine.cluster_model_collector import api
+from watcher.metrics_engine.cluster_model_collector import base
 
 LOG = log.getLogger(__name__)
 
 
-class NovaClusterModelCollector(api.BaseClusterModelCollector):
+class NovaClusterModelCollector(base.BaseClusterModelCollector):
     def __init__(self, wrapper):
         super(NovaClusterModelCollector, self).__init__()
         self.wrapper = wrapper

watcher/tests/decision_engine/strategy/strategies/faker_cluster_state.py

@@ -20,10 +20,10 @@ from watcher.decision_engine.model import hypervisor
 from watcher.decision_engine.model import model_root as modelroot
 from watcher.decision_engine.model import resource
 from watcher.decision_engine.model import vm as modelvm
-from watcher.metrics_engine.cluster_model_collector import api
+from watcher.metrics_engine.cluster_model_collector import base
 
 
-class FakerModelCollector(api.BaseClusterModelCollector):
+class FakerModelCollector(base.BaseClusterModelCollector):
     def __init__(self):
         pass