Get rid of pylint E0203 in filter_scheduler.py

by refactoring

Change-Id: I0b39293e7a7bb2154a6121ea8cdda8454dff2184

nova/scheduler/filter_scheduler.py

@@ -39,6 +39,7 @@ class FilterScheduler(driver.Scheduler):
     """Scheduler that can be used for filtering and weighing."""
     def __init__(self, *args, **kwargs):
         super(FilterScheduler, self).__init__(*args, **kwargs)
+        self.cost_function_cache = None
         self.options = scheduler_options.SchedulerOptions()
 
     def schedule_run_instance(self, context, request_spec,
@@ -294,7 +295,7 @@ class FilterScheduler(driver.Scheduler):
         """Returns a list of tuples containing weights and cost functions to
         use for weighing hosts
         """
-        if getattr(self, 'cost_function_cache', None) is not None:
+        if self.cost_function_cache is not None:
             return self.cost_function_cache
 
         cost_fns = []