Prevent the migration of VM with 'optimize' False in VM metadata

This patch adds the functionality to filter out VMs which have
metadata field 'optimize' set to False. This patch implements the
functionality for basic_consolidation strategy.

Change-Id: Iaf7b63e09534e4a67406e7f092242558b78c0bde
Partially-Implements: BP audit-tag-vm-metadata

watcher/decision_engine/strategy/strategies/base.py

@@ -39,6 +39,8 @@ which are dynamically loaded by Watcher at launch time.
 import abc
 import six
 
+from oslo_utils import strutils
+
 from watcher.common import clients
 from watcher.common import context
 from watcher.common import exception
@@ -264,6 +266,22 @@ class BaseStrategy(loadable.Loadable):
     def state_collector(self, s):
         self._cluster_state_collector = s
 
+    def filter_instances_by_audit_tag(self, instances):
+        if not self.config.check_optimize_metadata:
+            return instances
+        instances_to_migrate = []
+        for instance in instances:
+            optimize = True
+            if instance.metadata:
+                try:
+                    optimize = strutils.bool_from_string(
+                        instance.metadata.get('optimize'))
+                except ValueError:
+                    optimize = False
+            if optimize:
+                instances_to_migrate.append(instance)
+        return instances_to_migrate
+
 
 @six.add_metaclass(abc.ABCMeta)
 class DummyBaseStrategy(BaseStrategy):