Updated openstack/openstack
Project: openstack/nova  473bf13762c560352a1e4a7e2d8da1b607476b26

objects: restore some datetime field comments

The datetime coercion code in nova.objects.utils had a couple of
helpful comments which were lost when the code was refactored into
nova.objects.fields.

Change-Id: If23320dd8780a1947ad4d5946c55e1e8794b782f