Ensure the statistics are sorted

Sort the statistics by the period start date before
returning them, and restore the test that verifies
this behavior.

bug 1151345

Change-Id: I4f05ea049b7609cea9dab947738b7aabf3f062ef
Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>

ceilometer/storage/impl_mongodb.py

@@ -20,6 +20,7 @@
 
 import copy
 import datetime
+import operator
 import re
 import urlparse
 
@@ -519,7 +520,8 @@ class Connection(base.Connection):
             query=q,
         )
 
-        return [r['value'] for r in results['results']]
+        return sorted((r['value'] for r in results['results']),
+                      key=operator.itemgetter('period_start'))
 
     def get_volume_sum(self, event_filter):
         """Return the sum of the volume field for the events