aggregates: normalize timestamp
aggregates: normalize timestamp

This code is broken with Pandas 0.17 because the timestamps are not all
normalize. This patch fixes that and makes it also compatible with older
Pandas version.

Change-Id: Ie86bb58213a1fc5a25b80b25c330c5f271ebd4ea

leandroreox/gnocchi,leandroreox/gnocchi,sileht/gnocchi,gnocchixyz/gnocchi,sileht/gnocchi,gnocchixyz/gnocchi