Fix attachment IDs

[cloudartisan]

Fixes #40

The attachment resource is saving add-on name as add-on ID. This results in a change on every terraform plan because add-on name != add-on ID.

Terraform will perform the following actions:

heroku_addon_attachment.sumologic (new resource required)
       id:       "5382910f-92d4-4952-bc39-a92de2f87a72" => <computed> (forces new resource)
       addon_id: "sumologic-cubic-53500" => "1c7aa1ef-a6fc-47a4-abbb-b23512fc15a7" (forces new resource)
       app_id:   "sfdc-cb-rt-stg-ore" => "sfdc-cb-rt-stg-ore"
       name:     "SUMOLOGIC" => <computed>

Plan: 1 to add, 0 to change, 1 to destroy.

Note the add-on name sumologic-cubic-53500 has been saved in the state file as the add-on ID for the heroku_addon_attachment resource. It should have been saved as 1c7aa1ef-a6fc-47a4-abbb-b23512fc15a7 in the example above, not sumologic-cubic-53500.

[justincampbell]

The acceptance tests fail with this change. I ran:

$ TESTARGS="-run AddonAttachment" make testacc

I noticed the acceptance tests are also slightly incorrect. We create an app and an addon (attached to the app), and then create an addon_attachment to attach the addon to the app again. I think a better test would be to have 2 apps, 1 addon, and then attach the addon to the 2nd app.

[cloudartisan]

@justincampbell thanks for the feedback. Yesterday was the first time in my life I touched a line of Go and couldn't find any good instructions on contributing. I did run make test and didn't get any errors, so assumed all was well with the existing tests. I can take a look, maybe later today, when I have more time to try to understand the Go code of the test suite.

[joestump]

I suspect it's failing on tests where app_id is expected to be appName: https://github.com/terraform-providers/terraform-provider-heroku/blob/master/heroku/resource_heroku_addon_attachment_test.go#L21

[joestump]

I think the fix is to remove appName from the those test checks and use TestCheckResourceAttrSet instead of TestCheckResourceAttr.

[davidji99]

@cloudartisan I am closing this PR. I merged your change in another PR.