add Created field to images command

[bramalho]

In this PR we are adding the field Created to doctl images command. This is useful for us to manage our images/snapshots available for the autoscale group. Once this field is available in godo and DO API.

[loosla]

👋 Hey Bruno,
Thanks for an awesome change 🚀
Let's fix the integration tests and it should be good to go.

You can run integration tests with
doctl$ make test_integration

The changes like this

are needed in the files:
integration/droplet_backups_test.go
integration/droplet_snapshots_test.go
integration/image_create_test.go
integration/image_get_test.go
integration/image_list_application_test.go
integration/image_list_test.go

Feel free to add
"created_at": "2020-07-28T16:47:44Z" in a response in integration/droplet_backups_test.go and integration/droplet_snapshots_test.go. Here is the example.