device_attachment resource and data source don't function correctly

[wrgeorge1983]

currently they track the attachment task and not the raw state of whether or not a device is attached. This is problematic for a few reasons, most importantly that vManage cleans up these tasks after a few days, which ensures that terraform will always lose this state. Currently this results in a crash, which could be fixed, but even in the best case with this strategy it would involve re-applying device attachments any time terraform was run against an existing environment more that a couple days old