Nodes now have a `created_at` attribute * Base Node object has `created_at` which indicates the `datetime` the node was launched/started/created. * EC2, Digital Ocean, OpenStack fill this attribute. * Nodes at drivers that do not (yet) support it have `NoneType` as date. * Document changes. closes #698 [GITHUB-698] Signed-off-by: Allard Hoeve <allardhoeve@gmail.com>