add digitalocean_images datasource

[tdyas]

Use the datalist library to add a digitalocean_images datasource. Also update the digitalocean_image datasource to use the new common code.

This should supersede https://github.com/terraform-providers/terraform-provider-digitalocean/pull/79, although an open question is whether there should be a type attribute like in the DO API for applications and distributions.

[tdyas]

Open question: should distributions and application be broken out somehow in either or both the digitalocean_image and digitalocean_images datasources? (Trying to see if that idea in https://github.com/terraform-providers/terraform-provider-digitalocean/pull/79 should be pulled forward.)

[andrewsomething]

Great work! Two small changes needed to fix up the existing tests.

should distributions and application be broken out somehow

One slightly confusing thing to note here is that the type query parameter doesn't actually map onto the type attribute returned by the API:

Describes the kind of image. It may be one of "snapshot", "backup", or "custom". This specifies whether an image is a user-generated Droplet snapshot, automatically created Droplet backup, or a user-provided virtual machine image.

https://developers.digitalocean.com/documentation/v2/#images

So it would probably still be helpful to have an argument that chooses godo.Images.ListUser, godo.Images.ListApplication, godo.Images.ListDistribution. That also brings some performance gains. Doesn't necessarily need to block this PR

[tdyas]

So it would probably still be helpful to have an argument that chooses godo.Images.ListUser, godo.Images.ListApplication, godo.Images.ListDistribution. That also brings some performance. Doesn't necessarily need to block

How about source as the name of the parameter? Distinct from type but still conveys what it does.

[andrewsomething]

@tdyas 👍 Sounds good!

[andrewsomething]

👍 LGTM

Thank you yet again for another great contribution!