[digitalocean|compute] add created_at timestamp as attribute of server

* We receive this field from Digital Ocean, so all we need to do to use it is add it as an attribute and it just works
[digitalocean|compute] additional test coverage and some maintenance

* Add created_at field to mock response
* Assert that the created_at field is returned
* Update references to nonexistent images
Merge pull request #2515 from 12spokes/digitalocean-created-at

[digitalocean|compute] Add created_at timestamp as attribute of server