Add lxd_cached_image resource

[sl1pm4t]

This adds the lxd_cached_image resource. It can used to manage local
copies of remote images, set aliases etc.

I've named it lxd_cached_image to distinguish it from a future
resource I'm planning to create, that will allow users to create an LXD
image from a tarball.

[jtopjian]

Nice resource :)

With regard to aliases, copy_aliases, and copied_aliases, if I'm understanding it correctly, this is a common pattern emerging in some Terraform resources. I recently went through this with two OpenStack resources. The recommended approach was to treat one as an "input" and one as a computed read-only "output". So you could have aliases as the input and all_aliases as the output. (all_ being the suggested prefix).

In the Read function, you would no longer do a d.Set("aliases", x) since the user's configuration will be the source of truth. all_aliases, on the other hand, will be the API's source of truth. Depending on the results, the value of all_aliases might just equal aliases, or a combination of aliases and upstream-specified aliases.

Makes it a lot easier to handle these situations :)

And I think you could remove the copy_aliases attribute by doing this, too.

Thoughts?

[sl1pm4t]

Yes, the aliases are a bit of a PITA - and will probably go mostly unused. Especially if all the resources are managed with Terraform, there shouldn't be much need for the aliases.
However, having already coded up handling of the aliases I might as well keep it in place.
If I fall back to a aliases input that doesn't get updated on Read, then it would make it impossible to ever add or remove aliases right?!

copied_aliases is slightly different to all_aliases, in that it only includes the aliases that were defined in the remote LXD server at the time the image copy was created. If it were possible, I would make it an internal hidden attribute used only for the purposes of suppressing diffs.

I could also add all_aliases output that you mention, that reflects the merged set of aliases.

[jtopjian]

OK, I understand. The key here is the ability to opt-in to setting aliases. If it was mandated that downloaded/cached images received the upstream aliases, then the alias, all_aliases combo would make sense.

Give me a day or so to test this and poke at it. :)

[jtopjian]

This looks good to me -- everything does make sense.

I opened #44 which uses Terraform core's built-in set functions, but really it's six one way, half a dozen the other. I think playing around with the scenario of the aliases/copied_aliases can go on all day, so feel free to discard it. :)