Change default values from [] to None

Its well known fact that python default values initialized only once
at the stage if creation function. So if we call function with parameter 
foo=[] as default value and change foo.append("bar") at the next call of 
the function default value would be ["bar"]. It will save.

Change-Id: Iba9e55bc2d4dfd811ce7d068b6a9cceae7c000d6

ceilometerclient/v1/meters.py

@@ -18,8 +18,9 @@ import six
 from ceilometerclient.common import base
 
 
-def _get_opt_path(simple_params=[], **kwargs):
+def _get_opt_path(simple_params=None, **kwargs):
     l = []
+    simple_params = simple_params or []
     # get simple paramters
     for key in simple_params:
         val = kwargs.get(key)

ceilometerclient/v2/statistics.py

@@ -44,7 +44,10 @@ class StatisticsManager(base.Manager):
                 )
         return url_aggregates
 
-    def list(self, meter_name, q=None, period=None, groupby=[], aggregates=[]):
+    def list(self, meter_name, q=None, period=None, groupby=None,
+             aggregates=None):
+        groupby = groupby or []
+        aggregates = aggregates or []
         p = ['period=%s' % period] if period else []
         if isinstance(groupby, six.string_types):
             groupby = [groupby]