Allow specifying initial attachment name when creating addon.

[davidji99]

Resolves #227

[davidji99]

From what I experienced initially, the Heroku Platform API does two things:

• Does not return attachment.name in the either the POST or GET call to /apps/{app_id_or_name}/addons. It does show up in the UI:

So I am wondering if the provider needs to somehow use another endpoint to lookup if the attachment_name is set properly.

• Does not allow you to update the attachment name as the following error is returned when attempting to do so:

{
    "id": "invalid_params",
    "message": "Unknown params: attachment:name."
}

Therefore, the new heroku_addon.attachment_name attribute will be set to ForceNew as it cannot be updated, only set on initial creation.

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Questions to answer:

• What happens when someone has a heroku_addon_attachment resource that specifies a different name? Could this cause an infinite dirty plan?
• Should we rename heroku_addon.name and heroku_addon.attachment_name? At first look, the naming sounds reasonable since it's verbatim from the Platform API documentation.
• Any issues when running a new provider version on an existing tf configuration code base?