Sync updated_at and created_at of nova resources on initial create

According to the discussion in ML: http://lists.openstack.org/pipermail/openstack-dev/2016-June/098299.html

We should sync updated_at and created_at of nova resources on initial create as 'updated_at' as meaning 'the last time this record was modified'. For a new record,
the initial creation is also the last time it was modified. And this is how the
timestamps work for POSIX filesystems. When you create a file it sets the access/modify/change timestamps to the file creation time.