Allow specifying fields which API doesn't expose during import

[radeksimko]

As described in hashicorp/terraform-provider-aws#1807

It's currently non-intuitive/tricky to import resources which have fields that were passed as part of creation, but Read API doesn't expose the field anymore, so it's impossible for Terraform to reconstruct the full state.

One possible solution would be to either read those fields from the config (we AFAIK already read the provider config during imports) and copy values to state or let the user specify them as CLI flags, e.g.

$ terraform import aws_cloudformation_stack stackID -set-field=on_failure=DELETE

I'm creating the issue here in core as it's IMO a provider-wide problem impacting any resource which has create-only fields which API doesn't expose.

[BlessJah]

My understanding is that argument that is passed on creation, but cannot be read or changed later is either:

• hidden (e.g. volume encryption key that cannot be retrieved or changed)
• discarded (e.g. on_failure which only affects stack creation)

I do believe that my issue is caused by terraform-provider-aws not distinguishing those two cases. I do also believe that problem of non-readable, non-updatable attributes has to be resolved on case-by-case basis, not by exposing API/CLI flag to change them.