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>
9 files changed